CT Physics and Instrumentation

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A Metallic dental fillings often causes streak artifact during CT examinations of the head.

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A The image noise is equal to the standard deviation of pixel values within an ROI measurement of the image. ROI number 1 on the figure demonstrates the highest value for standard deviation (SD=2.87).

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A The limiting resolution of a particular CT scan is determined at a point on the graph where the signal frequency corresponding to a particular object has reached 10%. When the MTS is lower than 10% (0.1) the object is no longer resolved.

The mathematics of back projection applied to image reconstruction in CT is credited to: A. Alan Cormack B. Raymond Damadian C. Geofrey Hounsfield D. Frank Lauterbur E. Perry Sprawls

A. Alan Cormack

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A. bone According to the MTF diagram, the bone algorithm has a resolution of approximately 12 lp/cm. The standard of algorithm and smooth algorithm are approximately 10 and 7 lp/cm, respectively.

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B Presence of metal within the patient causes a streak artifact on the image. Their effect occurs when the dense metal absorbs a large amount of radiation, interfering with the signal produced.

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B The artifact present in the figure most likely represents an out-of-field artifact. This relatively large patient was incorrectly positioned, and a portion of the anatomy lies outside the scanned field of view. This improperly centered anatomy interferes with the reference detectors, thus causing a streak artifact near the unscanned area.

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B The longitudinal spatial resolution is usually qualified by the extent of broadening that occurs to the slice sensitivity profile during volumetric acquisition. The section width slice for a volumetric acquisition may be graphically represented as a slice sensitivity profile (SSP).

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B The width of the window used to display a CT image of the abdomen should be within the range of 350 to 600 HU. This range allows for excellent visualization of the soft tissue structures.

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B. 1 and 2 only Streaking edge gradient artifacts may be reduced with increases in the beams average photon energy. Increases in kVp and filtration would accomplish this. This type of artifact can also be reduced with a decrease in reconstructed section width, which helps maximize the edge gradient effect. A decrease in partial volume averaging may lessen the streaking that is apparent from edge gradient inconsistencies.

Place the following in chronological order 1. CT 2. electron beam CT (EBCT) 3. multiseciton computed tomography 4. Spiral CT A. 1,2,3,4 B. 1,2,4,3 C. 1,3,2,4 D. 1,4,2,3 E. 1,3,4,2

B. 1,2,4,3

Compared to projection radiography, conventional tomography results in improved contrast resolution because: A. Imaging time is reduced B. Out of plane tissues are blurred C. Tissues are superimposed D. Precise beam collimation is employed E. The x-ray beam is selectively filtered

B. Out of plane tissues are blurred

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C Beam hardening occurs as low-energy xray photons are absorbed while the beam passes through the patient. The average photon energy of the beam increases along the path and may result in a loss of system uniformity.

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C Region of interest measurements may be made by superimposing a cursor over an area and instructing the computer to average the CT numbers included within the region.

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C The CT number for water has an average value of 0.

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C The edge gradient effect occurs when the CT X-ray being passed through areas of abrupt changes in density that are represented by high spatial frequencies. This type of streak artifact commonly occurs at the interface of dense bone and soft tissue in anatomic areas such as the brain.

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C The level chosen for a given window setting should correspond to the average density value of the tissue of interest. Areas of soft tissue, such as the brain, are often displayed at window levels of approximately +50 HU.

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C The limiting resolution of a particular CT scan is determined at a point on the graph where the signal frequency corresponding to a particular object has reached 10% (MTF = 0.1).

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C The section width (slice) for volumetric acquisition may be graphically displayed as the slight sensitivity profile (SSP). The effective section width is defined as the full-width at half-maximum (FWHM) of the SSP. The full-width at half-maximum is labeled in the figure as C. At this point of the graph, the section width of B is wider than that of A.

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C. 0.206 At the range of photon energy is employed during most CT examinations, water exhibits an approximate attenuation coefficient value 0.206.

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C. coronal MRP images built from an axial acquisition An isotropic axial acquisition of the paranasal sinuses from the hard palate superiorly through the frontal sinuses avoids artifact from the dental fillings while providing excellent multiplayer reformations (MPRs) in any plane. Direct coronal acquisition may not be required because the detail provided by the reformatted images is usually sufficient. This technique also aids in reducing patient radiation dose.

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D Noise is most commonly measured by scanning a water filled phantom with a consistent set of technical factors. The image noise is equal to the standard deviation of pixel values within a ROI measurement of the image. Uniformity may also be evaluated by positioning several ROI measurements at different locations along the center and periphery of the image. The CT values should not differ by more than 2 HU from one location to another.

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D The out-of-field artifact present in this image could be easily reduced by properly centering the patient within the scan field of view.

Math formula question #122 pg 143

D This important exponential equation illustrates the mathematical relationship between the primary beam, see object thickness (density), and the radiation measured by each detector in each calculation of the linear attenuation coefficient.

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D Water has a CT number at or near zero. This area in the ROI in the figure most likely represents a hepatic cyst.

How much anatomy (lengthwise) will be covered in a helical scan when the following parameters are selected: 15 seconds total acquisition time, .5 seconds gantry rotation time, 2mm slice thickness, 4 slices per rotation , pitch of 1.5? A. 40mm B. 90mm C. 160mm D. 360mm

D. 360mm Pitch x total acquisition time x 1/rotation time x (slice thickness x slices per rotation) = amount of anatomy covered 1.5 x 15s x 1/0.5s x (2mm x 4) = 360mm Pg 55 (CT for techs)

Which of the following image modalities are likely to have less scatter radiation affecting the image? A. Fluoroscopy B. Projection of radiography C. Conventional tomography D. CT E. Radioisotope emission tomography

D. CT

Computed tomography results in improved contrast resolution because A. Digital techniques are employed B. Tissue superimposition is reduced C. Tissues are superimposed D. Precise beam collimation is employed E. KVp is significantly increased

D. Precise beam collimation is employed

A CT system measures the average linear attenuation coefficient of a voxel of tissue to be 0.008. The linear attenuation coefficient of water for this scanner equals 0.181. The CT number assigned to the pixel representing the voxel of tissue equals: a. -956HU b. -173HU c. +44HU d. +1044HU

a. -956HU The CT number of the pixel maybe calculated by subtracting the linear attenuation coefficient of water from the linear attenuation coefficient of the tissue within the voxel (0.008 - 0.181 = -0.173). This number is divided by the linear attenuation coefficient of water (-0.173 / 1.81 = -0.956. The quotient is multiplied by contrast factor of 1000 to yield the value of the pixel in Hounsfield units.

Two adjacent pixels are measured to have a difference of 1HU. This amounts to a tissue density difference of approximately: a. 0.1% b. 1.0% c. 10% d. 25%

a. 0.1% Pixels that differ by only 1 Hounsfield unit represent tissue whose attenuation coefficients differ by only 0.1%.

An image is produced with a 1mm aperture size and is displayed using a 512x512 matrix and a 15 cm DFOV. The voxel dimension for this image would be: a. 0.29 mm x 0.29 mm x 1.0 mm b. 2.9 mm x 2.9 mm x 1.0 mm c. 3.4 mm x 3.4 mm x 1.0 mm d. 0.29 mm x 0.29 mm x 1.0 cm

a. 0.29 mm x 0.29 mm x 1.0 mm The pixel dimensions may be calculated by dividing the DFOV by the matrix size. This two-dimensional pixel size is then multiplied by the length of the voxel.

A standard Ct image reconstructed in a 512x512 matrix has a digital size of approximately: a. 0.5 MB b. 3.0 MB c. 5.0 MB d. 10.0 MB

a. 0.5 MB Each CT image reconstructed on a 512x512 matrix is approximately 0.5 MB in size.

Filament sizes for modern CT x-ray tubes range between: a. 0.5 and 1.2 mm b. 2.0 and 3.0 mm c. 5.5 and 7.8 mm d. 25.0 and 31.5 mm

a. 0.5 and 1.2 mm Modern CT system x-ray tubes utilize dual-filament systems with the focal spot sizes ranging between 0.5 and and 1.2 point two mm in diameter.

An MDCT image is reconstructed using a 512 x 512 matrix in a DFOV of 38 cm. If the detector collimation is set to a section width of 1.25 mm, what is the volume of each voxel? a. 0.69 mm^3 b. 0.93 mm^3 c. 1.26 mm^3 d. 1.68 mm^3

a. 0.69 mm^3 The linear dimension of the pixel must first be calculated by dividing the DFOV, in millimeters, by the matrix (380 mm/512 = 0.74mm). This linear pixel dimension is squared to yield the pixel area in mm^2 (0.74 x 0.74 = 0.55mm^2). The volume of the voxel may be calculated by multiplying the pixel area by the section width 0.55 mm^2 x 1.25 mm = 0.69 mm^3)

Which of the following azimuth settings is used to produce an AP localizer image? a. 0° b. 90° c. 180° d. 270°

a. 0° The azimuth setting the relationship between the x-ray tube and detectors during scout or localizer production. A 0° azimuth describes the situation in which the x-ray tube and detectors did not rotate but remain above and below the patient, providing a frontal projection.

Which of the following mathematical techniques is used for the reconstruction of volumetric MDCT images? a. 180 degree interpolation b. 360 degree interpolation c. back projection d. Fourier reconstruction

a. 180 degree interpolation The 180 degree linear interpolation technique is commonly used for current MDCT image reconstruction. This method interpolates data acquired at a distance only 180 degrees away from the location of the reconstructed slice.

Which of the following mathematical techniques is used for the reconstruction of volumetric MDCT images? a. 180° interpolation (180LI) b. 360° interpolation (360LI) c. back projection d. Fourier reconstruction

a. 180° interpolation (180LI) The 180° interpolation technique (180LI) is commonly used for current MDCT image reconstruction. This method interpolates data acquired at a distance of only 180° away from the location of the reconstructed slice.

Which of the following is used to remove image blurring during the back projection method of CT image reconstruction? 1. z-filtering 2. convolution kernel 3. Feldlamp-Davis-Kress (FKD) algorithm a. 2 only b. 3 only c. 1 and 2 only d. 2 and 3 only

a. 2 only During CT image reconstruction using the back-projection method, the data are manipulated with a convolution kernel or algorithm to remove image unsharpness (blurring). This method is called the filtered back-projection. Z-filtering is the mathematical process that allows MDCT images to be reconstructed at any point along the acquired volume.

MSCT systems are typically capable of differentiating adjacent objects with attenuation differences as small as: a. 3 HU b. 10 HU c. 25 HU d. 60 HU

a. 3 HU Each 1.0% difference in contrast between adjacent objects amounts to a difference in pixel value of approximately 10 HU. MSCT Systems are typically capable of differentiating adjacent objects with attenuation differences as small as 3 HU.

Which of the following correctly illustrates the central volume principle of cerebral perfusion? a. CBF = CBV/MTT b. CBF = MTT/CBV c. MTT = CBV CBF d. CBF = CBV MTT

a. CBF = CBV/MTT The central point principle is described by the formula CBF = CBV/MTT. It illustrates the relationship of cerebral blood flow as a factor of the volume of blood within a given area of brain tissue in the time it takes for that blood to pass through the area. Reduced CBD may indicate ischemia due to stroke or other brain abnormalities. like Speed=Miles/Hour, Flow = volume/time (cerebral blood volume/mean transit time)

In 1917, Austrian mathematician _____ proved that it was possible to reconstruct a three dimensional object from the infinite set of all of its projections. a. Radon b. Tsien c. Bracewell d. Hounsfield

a. Radon J. Radon, an Austrian mathematician, was responsible for some of the earliest scientific research for the Reconstruction principles used in CT. In 1917 he proved that it was possible to build an image of an object through the use of an extremely large set of its projections.

Which of the following manipulations involves the use of image data? a. adjusting the width in level of the window setting b. decreasing the DFOV c. changing the algorithm selection d. increasing the matrix size

a. adjusting the width in level of the window setting The adjustment of the window width or level is a computer manipulation of image data. The image has already been reconstructed from scan or "raw" data. The window changes only the range of pixel values that will be assigned a shade of gray. You can also use image data to magnify (different than DFOV) as well as take ROI/distance measurements on an image because it only changes how the image is viewed. Adjustments in the algorithm, DFOV, or matrix require the raw data so that the image may be retrospectively reconstructed.

The polyenergetic x-ray beam emitted from a CT X-ray tube is susceptible to artifacts due to the phenomenon known as: a. beam hardening b. density bloom c. kerma d. partial volume

a. beam hardening The polyenergetic CT x-ray consists of photons of varying energy. As the beam traverses the patient, low energy photons are absorbed first, increasing the average intensity of the beam as it travels along its path. This change, referred to as beam hardening, can have an artifactual result on the CT image.

The electronic combination of signals from adjacent detectors to form a CT image is called: a. binning b. interpolation c. z-filtering d. partial volume averaging

a. binning Binning refers to the electronic combination of signal from the adjacent detectors to form a reconstructed slice that is thicker than the individual detector width. For example, transmission data from four adjacent 1.25 mm detectors is combined to produce a CT section 5.0 mm thick.

The smallest unit of information used in the binary language of computers is the: a. bit b. chip c. base d. byte

a. bit The bit is the smallest unit if info within the binary system. Its name is derived from the term "binary digit" and can appear as either a number 1 or 0. A sequence of eight bits constitutes a byte.

Which of the following symptoms indicates a vagal reaction to an iodinated IV contrast agent? a. bradycardia and systolic pressure below 80 mm Hg b. bradycardia and diastolic pressure above 90 mm Hg c. tachycardia and systolic pressure below 80 mm Hg d. tachycardia and diastolic pressure above 90 mm Hg

a. bradycardia and systolic pressure below 80 mm Hg A vagal reaction to iodinated IV contrast must be recognized early so that proper and initial treatment is provided. A combination of bradycardia (heart rate below 50 bpm) and hypotension (systolic pressure below 80 mm Hg) indicates a vagal reaction.

The process of applying a mathematical filter to remove blurring from the reconstructed CT image is termed: a. convolution b. interpolation c. iteration d. z-filtration

a. convolution After the back-projection step is performed, the process known as convolution is applied to reduce image unsharpness. The acquisition of an extremely large number of projections for each gantry rotation results in a detailed but relatively blurry digital representation of the object. An algorithm, or convolution kernel, acts as a mathematical filter, modifying the ray sum data and removing the unwanted blurring effect of the back-projection. The combo of back-projection and the convolution algorithm is called "filtered back projection" Z-filtration is the mathematical process that allows MDCT images to be reconstructed at any point along the acquired volume.

Which of the following is a manifestation of the partial volume artifact? a. cupping b. hounsfield bar c. aliasing d. streaking

a. cupping The cupping artifact that occurs at the superior portion of the skull is another form of partial volume artifact. The dense skull table averages with the low attenuated brain tissue, causing an artifact by which the brain parenchyma may appear abnormally dense.

Quality control measurements to test the accuracy of the CT scanner's calibration should be performed: a. daily b. weekly c. monthly d. annually

a. daily It is important that the calibration of a CT scanner be checked daily by the operator. CT units should be calibrated with their reference CT number for water at approximately zero. The CT number for air should be at approximately -1000.

In an MSCT system, which of the following technical parameters determines the reconstructed section width? a. detector collimation b. number of data channels c. beam collimation d. detector pitch

a. detector collimation With MSCT, detector collimation determines the width of the reconstructed section. By electronically adjusting the detector dimension, the operator can control the width of the x-ray beam contributing to a reconstructed section. Beam collimation no longer directly controls section width.

The number, length, and organization of the individual detector elements in an MSCT system are referred to as the: a. detector configuration b. array pitch c. CT generation d. detector pitch

a. detector configuration The configuration of the x-ray tube to the detectors determines the scanner generation. Pg 18 (ct for techs)

Which of the following terms best describes the streaking artifact from incision staples? a. edge gradient b. Gibbs phenomenon c. density gradient d. partial volume

a. edge gradient The edge gradient artifact manifests as a streaking artifact at areas of extremely high density interfaces.

For a given CT acquisition, the calculated mAs applied to each reconstructed slice maybe referred to as the: a. effective mAs b. peak mAs c. absorbed mAs d. constant mAs

a. effective mAs Effective mAs describes the calculated mAs per acquired sliced. The primary influencing factor for effective mAs is table speed, which is in important component controlled by the selected detector pitch. Detector pitch chosen by the CT operator directly affects the speed at which the patient travels through the gantry. The mAs divided by the pitch yields the effective mAs for each slice in a given CT acquisition.

During the volumetric CT acquisition, the slice sensitivity profile graphically represents the: a. effective section width b. dose profile c. beam width d. detector collimation

a. effective section width The effective section width corresponds to the SSP of the reconstructed section in consideration of the widening that occurs during helical data acquisition.

The contrast resolution of a CT scanner is NOT related to which of the following? a. focal spot size b. section width c. reconstruction algorithm d. signal-to-noise ratio

a. focal spot size The contrast resolution of a CT scanner depends on several factors, including section width, algorithm selection, detector sensitivity, and noise. The focal spot size is a geometric factor that influences the spatial resolution of a CT scanner.

Which of the following would be considered the best method to reduce respiratory motion on the CT image? a. good patient technologist communication b. reduced scan times c. immobilization devices d. glucagon administration

a. good patient technologist communication Thorough communication between the technologist and the patient is vital in ensuring superior exam quality. If the patient has been informed of the exam process, his anxiety may be reduced and cooperation improved. During all CT procedures, the patient must be instructed to hold still an effort to reduce motion artifact on the CT images.

CT numbers are usually provided in the form of: a. hounsfield units b. EMI numbers c. Cormacks d. μ

a. hounsfield units The CT number is a relative value based on the attenuation that occurs within a voxel of tissue. The Hounsfield unit is used for this value.

As the attenuation of a volume of tissue decreases, the transmitted intensity of the CT X-ray beam: a. increases b. remains the same c. decreases d. increases to a peak value and then rapidly decreases

a. increases The transmitted intensity of a CT x-ray beam and the attenuation of the tissue image are inversely related. As the tissue begins to attenuate less radiation, the transmitted intensity of the beam increases. Areas of less dense tissue allow more radiation to pass onto the detectors, and vice versa.

Where is the high-frequency generator often located in a modern CT scanner? a. inside the gantry b. just outside the scan room c. beneath the CT table d. inside the operator's console

a. inside the gantry The high-frequency used to produce the three-phase power used in modern CT scanners is located inside the gantry. It may be positioned in a corner of the gantry or fixed to the rotating tube assembly.

Which of the following is used to archive a hard copy of a CT image? a. laser film b. PACS c. CD-ROM d. LAN

a. laser film Storage on film is considered to be a hard copy. The digital storage of CT images on PACS, CD-ROM, MOD, and so on, is referred to as soft copy.

The ability of an object to attenuate the x-ray beam is assigned a value known as the: a. linear attenuation coefficient b. hounsfield value c. CT number d. ray sum

a. linear attenuation coefficient The ability of an object to attenuate the x-ray beam is assigned a value turned the linear attenuation coefficient (μ).

The term used to describe the relationship between the linear attenuation coefficient of an object and the calculated CT numbers is: a. linearity b. uniformity c. quantum noise d. spatial resolution

a. linearity Linearity describes the relationship between the CT number and actual linear attenuation coefficients of an object. It is used to measure the accuracy of a CT scanner. pg 109

Compared with conventional radiography, computed tomography produces diagnostic images with better: a. low contrast resolution b. spatial resolution c. minute detail d. patient radiation dose reduction

a. low contrast resolution Computed tomography has better low contrast resolution over conventional radiography. The CT system is extremely sensitive to small changes in tissue density and also remove the problem of superimposition, both leading to greater contrast resolution.

The type of MSCT detector array that contains multiple rows of detector elements, each of the same length, is called a(n): a. matrix array b. adaptive array c. hybrid array d. stationary array

a. matrix array A uniform matrix array consist of multiple detectors in the longitudinal direction, each with the same dimensions.

A picture archival and communications system (PACS) is connected with the imaging modalities and other peripheral devices on a computerized communication system called a: a. network b. web c. radiology information system d. hospital information system

a. network The computer network is a communication system designed to facilitate the transfer of data between computers. A PACS works on a network, which connects each of the involved imaging modalities, viewing stations, printers, and so on.

Automatic tube current modulation (ATCM) software is used in current MDCT systems to control: a. patient radiation dose b. cardiac motion c. timing of the contrast agent bolus d. tissue perfusion assessment

a. patient radiation dose Automatic tube current modulation (ATCM) programs adjust the mA throughout an acquisition to reduce patient radiation dose to a minimum. ATCM automatically alters the applied mA on the basis of a predetermined noise index that is acceptable for appropriate image quality.

First generation CT scanners possess which of the following characteristics? a. pencil thin x-ray beam b. Silver halide detectors c. rotate-rotate geometry d. nutating detector array

a. pencil thin x-ray beam The x-ray beam of a first-generation CT scanner was highly collimated to the size of a single detector. It was often referred to as a "pencil beam".

The bit depth of a digital imaging system defines the number of information bits contained within each: a. pixel b. matrix c. Axial image d. Multiplanar reformation

a. pixel The bit depth of a digital imaging system defines the number of information bits contained within each pixel. This parameter ultimately controls the total range of CT values that may be assigned to a given pixel. For example, a bit depth of 12 results in 4096 possible CT values (2^12 = 4096).

The implementation of corrective actions to improve any identified performance inadequacies of the CT system is referred to as: a. quality control b. uniformity c. quality assurance d. linearity

a. quality control Quality control refers to the implementation of corrective actions to improve any identified performance inadequacies of the CT system found through quality assurance procedures. Quality assurance is the measurement of the scanner's performance through quality testing procedures and evaluation of the test results.

Which of the following is the most common type of noise found in the CT image? a. quantum noise b. electronic noise c. artifactual noise d. filter noise

a. quantum noise The most common cause of CT image noise is the fluctuation in the number of x-ray photons measured by the detectors. When a CT scanner attempts to reconstruct an image from an insufficient amount of transmitted radiation measurements, quantum noise occurs.

Retrospective reconstruction requires the use of _____ data to later generate a new image. a. raw data b. image data

a. raw data Raw data includes all measurements obtained from the detector array. Once a Hounsfield number is assigned to each pixel, image data can be formed. Image data requires about one-fifth of the computer space needed for raw data. Image data can be used to take measurements like H numbers and standard deviation and distance, but anything not seen on the image is unavailable for analysis Pg 25 (CT for techs).

Which of the following technical adjustments would result in a decrease in the signal-to-noise ratio? a. reducing the section width b. decreasing the detector pitch c. selecting a smooth algorithm over a bone algorithm d. increasing the display field of view

a. reducing the section width Any decrease in the voxel dimension increases noise or decreases the signal-to-noise ratio.

A high-resolution comb is utilized by an MSCT detector array in an effort to reduce: a. scatter radiation b. patient radiation dose c. low spatial frequency signal d. high spatial frequency signal

a. scatter radiation Post patient collimation occurs through a high resolution comb placed over the detector array. Functioning like a grid, it removes unwanted scatter radiation and off axis photons that results from the more divergent nature of the MSCT beam.

The full width at half maximum (FWHM) of a CT scanner is used to describe: a. spatial resolution b. contrast resolution c. noise d. calibration accuracy

a. spatial resolution Spatial resolution of a CT scanner can be measured by studying the amount of blurring that occurs around a point within the CT image. Known as the point spread function (PSF), this image unsharpness may be graphically represented. The spatial resolution can then be quantified by measuring the graph at half its maximum value. This measurement, called the full width at half maximum (FWHM), is used to illustrate the spatial resolution of a CT scanner.

The MTF of a CT system is used to evaluate which of the following components of CT image quality? a. spatial resolution b. signal to noise ratio c. temporal resolution d. contrast resolution

a. spatial resolution The MTF of a CT system is an objective measurement of a system's in-plane spatial resolution. By evaluating an MTF curve of a particular image, the operator can assess the spatial resolution of a CT system and appraise changes in spatial resolution due to adjustments in technical factors.

Segmenting the data acquisition process into separate components of smaller rotation angles may improve which of the following components of CT image quality? a. temporal resolution b. longitudinal spatial resolution c. contrast resolution d. in-plane spatial resolution

a. temporal resolution The controlling factors of the temporal resolution of a CT system are the gantry rotation speed and reconstruction method. Temporal resolution can be improved by segmenting the data acquisition process into separate components of smaller rotation angles. Single-segment or half-scan acquisitions reconstruct data obtained from half of the rotation time. For example, this type of segmentation would yield a 250-msec temporal resolution from a 0.5-sec gantry rotation time. pg 107

When one is viewing a maximum intensity projection (MIP) image, each pixel represents: a. the maximum attenuation occurring within the voxel b. the average attenuation occurring within the voxel c. the minimum attenuation occurring within the voxel d. all attenuation occurring within the voxel above a set threshold value

a. the maximum attenuation occurring within the voxel MIP images display only the maximum pixel value along a ray traced through the object to the viewers assumed perspective in front of the viewing monitor. As the viewer faces a MIP image, each pixel represents the maximum attenuation that occurs in the associated voxel.

An increase in the sampling rate during CT data acquisition corresponds to an increase in: a. views per rotation b. patient radiation dose c. Image noise d. Matrix size

a. views per rotation The sampling rate may be quantified as views per rotation (VPR). As the sampling rate rises, the number of views per rotation increases.

The CT number is a relative value that standardizes the attenuation value of a tissue by comparing it with the attenuation of: a. water b. air c. bone d. a standard Plexiglass phantom

a. water

Which of the following types of gases is commonly used for gas ionization CT detectors: a. xenon b. cadmium tungstate c. helium d. nitrogen

a. xenon Because of its high atomic number and relative stability, xenon gas is commonly used in gas ionization CT detectors.

A CT scanner with a limiting resolution of 15 lp/cm can resolve an object as small as: a. 0.1 mm b. 0.3 mm c. 0.6 mm d. 1.0 mm

b. 0.3 mm The minimal object size the CT scan can resolve may be calculated by taking the reciprocal value of the scanners limiting resolution. The reciprocal of 15 lp/cm is 1/15 lp/cm. This is equivalent to 10/15 lp/mm, which equals 0.6 mm/lp. Because a line pair (lp) is equivalent to a line and the space adjacent to it, this value maybe divided in half to provide the minimum object that the scanner can resolve: 0.3 mm.

A CT image is reconstructed using a 512x512 matrix in the display field of view of 17 cm. What is the linear dimension of each pixel? a. 0.033 mm b. 0.33 mm c. 3.01 mm d. 30.1 mm

b. 0.33 mm The dimension of a pixel maybe calculated by dividing the field of view by the matrix size. The display field of view of 17 cm must first be converted into 170 mm. This is then divided by 512 mm for a pixel dimensions of 0.33 mm.

A CT scanner capable of producing an image that is a perfect reproduction of the actual anatomic section is said to have an MTF of: a. 0 b. 1 c. 10 d. 100

b. 1 The modulation transfer function of a CT scanner measures the ability of the system to faithfully reproduce the area of anatomy. It can be thought of as a comparison of the actual anatomy with the image produced. If the two are exactly alike, the MTF of the scanner is 1. If the image produced contains no useful information, the MTF is 0. The MTF of most CT scanners fall somewhere in between.

The use of a wider window width can improve an image when: 1. metallic artifacts are present 2. the patient is obese 3. increased contrast is needed a. 1 only b. 1 and 2 only c. 1 and 3 only d. 1, 2, and 3

b. 1 and 2 only Because wider window width settings decrease image contrast, they suppress the display of noise on an image. For this reason, it is common practice to widen the window width when patients are obese or when there are metallic artifacts. Narrow widths provide greater density discrimination and contrast, making it ideal for scans of the brain where differentiation between white and gray matter are useful. Pg 35 (CT for techs)

Which of the following may be considered as advantages of helical CT over conventional CT? 1. reduced scan time 2. reduction of misregistration artifacts 3. reduction in patient dose a. 1 only b. 1 and 2 only c. 1 and 3 only d. 1, 2, and 3

b. 1 and 2 only Helical CT offers many advantages over conventional CT. Complete anatomical areas are scanned in reduced times owing to continuous acquisition and no interscan delay. Because no delays are needed, entire volumes of information may be obtained, often during only one breath-hold. This reduces misregistration artifacts associated with inconsistent patient breathing. There is no reduction in patient radiation dose with the use of spiral CT.

Which of the following technical adjustments maybe employed to reduce step artifact in a multiplanar reformation CT image? 1. acquisition of an isotropic dataset 2. overlapping section increment 3. increase in pitch a. 1 only b. 1 and 2 only c. 2 and 3 only d. 1, 2, and 3

b. 1 and 2 only In modern MSCT systems, the highest quality 3-D and multiplanar reformation (MPR) images are obtained with the acquisition of an isotropic or near isotropic dataset. The other equally important piece is an overlapping dataset, for which the reconstruction interval is less than the section width

Which of the following quality control test should be performed daily on the CT scanner? 1. check CT number calibration 2. examine noise levels (Standard deviation) of a water phantom 3. test accuracy of laser localization device a. 1 only b. 1 and 2 only c. 1 and 3 only d. 1, 2, and 3

b. 1 and 2 only Quality control test for CT numbar accuracy and noise level should be performed daily on CT scanners. The laser localization system of a CT scanner needs to be checked only once per year.

Soft copies of CT images are stored on which of the following media? 1. hard drives 2. magnetic optical discs 3. laser film a. 3 only b. 1 and 2 only c. 1 and 3 only d. 2 and 3 only

b. 1 and 2 only Storage of CT images on digital file format is referred to as soft copies. Hardcopies of CT images are those stored on laser film.

Which of the following would serve to increase spatial resolution of a CT exam using a large focal spot size, 5-mm sections, and a 512x512 matrix? 1. change to small focal spot size 2. acquire 2.5 mm sections 3. reconstruct images in a 320x320 matrix a. 1 only b. 1 and 2 only c. 1 and 3 only d. 1, 2, and 3

b. 1 and 2 only The spatial resolution of a CT examination can be improved with the use of small focal spots, narrow sections, and large matrices.

The technical factor(s) necessary for the acquisition of an isotropic dataset is/are: 1. thin detector collimation 2. high mA setting 3. small DFOV a. 1 only b. 1 and 3 only c. 2 and 3 only d. 1, 2, and 3

b. 1 and 3 only Isotropic voxel's have equal dimensions along the X, Y, and Z axes. Use of a thin reconstructed section within a small DFOV minimizes each voxel dimension.

An MDCT images reconstructed using a 512x512 Matrix in the display field of view of 44 cm. If the detector collimation is set to a section width of 2.5 mm, what is the volume of each voxel? a. 0.21mm^3 b. 1.85mm^3 c. 2.15mm^3 d. 3.36mm^3

b. 1.85mm^3 The linear dimension of the pixel must first be calculated by dividing the DFOV, in mm, by the matrix. This linear pixel dimension is squared to yield the pixel area, in mm^2. The volume of the voxel maybe calculated by multiplying the pixel area by the section width. 440 / 512 = 0.859375 0.859375^2 = 0.73852 0.73852 x 2.5 = 1.846

A 16-slice MSCT system utilizes an adaptive array of 24 detectors, each ranging in size from 0.625 to 1.25 mm. What is the maximum number of sections the system can acquire with each rotation of the gantry? a. 8 b. 16 c. 32 d. 64

b. 16 The number of data channels controls the number of sections the scanner can simultaneously acquire with each gantry rotation. a 16-slice system possesses 16 data channels and is capable of acquiring 16 images per gantry rotation. The configuration of the detector array with regard to the size of individual detector elements controls the possible section widths for each reconstructed CT image.

Which of the following steps could be taken to reduce the artifact from metal teeth fillings? 1. reduce section thickness 2. angle gantry around feelings 3. decrease kVp a. 1 only b. 2 only c. 1 and 2 only d. 2 and 3 only

b. 2 only

A 64 slice MSCT system employs an array of 64 detectors, each with a dimension of 0.625 mm. What beam collimation is required to expose the middle 32 detectors of the array to transmitted x-radiation? a. 10 mm b. 20 mm c. 32 mm d. 64 mm

b. 20 mm The selected detector configuration for a given acquisition determines the necessary beam collimation. The selected 32 detectors, each with an individual dimension of 0.625 mm, would require a beam collimation of 20 mm to expose each to transmitted radiation (32 x 0.62 = 20 mm).

What is the display field of view used for a 320x320 Matrix image with the pixel dimension of 0.75 mm x 0.75 mm? a. 12 cm b. 24 cm c. 36 cm d. 48 cm

b. 24 cm The display field of view maybe calculated by multiplying the pixel dimension by the matrix size. In this example, one side of the pixel measures 0.75 mm and the matrix used is 320x320. The DFOV is 240 mm or 24 cm.

The size of the CT image displayed on a computer monitor can be enlarged by increasing the: 1. DFOV 2. SFOV 3. image magnification a. 1 only b. 3 only c. 1 and 2 only d. 1 and 3 only

b. 3 only Increasing the display field of view (DFOV) for a reconstructed CT image decreases its displayed size. Electronically magnifying the image on the display system results in a larger displayed image but has no effect on pixel or voxel dimensions.

For a single-slice CT (SSCT) system, which of the following statements regarding pre-patient collimation is true? 1. an increase in pre-patient collimation increases patient radiation dose 2. pre-patient collimation is used to focus radiation through the section of interest 3. pre-patient collimation directly controls slice thickness a. 2 only b. 3 only c. 1 and 2 only d. 2 and 3 only

b. 3 only Pre-patient collimation influences the slice thickness by reducing the size of the primary beam. This is accomplished with the use of lead shutters that absorb the outer margins of the primary beam. Increases in pre-patient collimation diminish patient radiation dose by reducing the number of x-rays photons that reach the patient.

The approximate inherent filtration of the CT x-ray amounts to an aluminum equivalent of: a. 1.2 mm b. 3.0 mm c. 5.8 mm d. 10.0 mm

b. 3.0 The tube housing, cooling oil, and so on, constitute the inherent filtration, which amounts to approximately 3.0 mm of aluminum equivalent filtration.

The human eye is capable of differentiating a approximately_____ shades of gray. a. 10 - 25 b. 60 - 80 c. 100 - 120 d. 250 - 256

b. 60 - 80 The human eye is capable of differentiating only about 60 - 80 individual shades of gray. The 256 shades shades utilized in CT are therefore sufficient to represent the total range pixel values possible in a 12-bit system.

The average photon energy of the primary beam of a CT scanner operating at a tube potential of 120 kVp is approximately: a. 50 keV b. 70 keV c. 100 keV d. 120 keV

b. 70 keV The average photon energy of the primary beam used in CT is approximately 70 keV. The average photon energy of any radiographic primary beam is typically 30% to 40% less than the applied kilovoltage. The average photon energy of the CT beam is increased through beam filtration.

Which of the following is not an iterative method of CT image reconstruction? a. point-by-point correction b. Fourier transform c. simultaneous reconstruction d. ray-by-ray correction

b. Fourier transform The iterative methods a CT imagery construction include simultaneous reconstruction, ray-by-ray correction, and point-by-point correction. The analytic methods of CT image reconstruction include the filtered back-projection and the Fourier transform method. These techniques are called analytic because they utilize precise formulas for image reconstruction.

Which of the following abbreviations is used to identify a computerized network that stores, retrieves, communicates, and displays digital medical images? a. WORM b. PACS c. DICOM d. HIPAA

b. PACS

The symbol "x" in the formula used to calculate the linear attenuation coefficient identifies: a. Euler's constant b. absorber thickness c. the unknown d. transmitted photons

b. absorber thickness The Lambert-Beer law, is used to calculate the attenuation coefficient of the volume of material. The symbol "x" represents the thickness of the absorber attenuating the radiation.

Which of the following terms is used to describe a set of rules for solving a mathematical problem? a. reconstruction b. algorithm c. function d. array

b. algorithm An algorithm may be defined as a set of rules or steps used to solve a mathematical problem. The programs used by the CT computer to reconstruct the image are often referred to as algorithms

Before a CT image can be reconstructed by a computer, the transmission signal produced by the detectors must be converted into numerical information by: a. kernel b. analog to digital converter c. array processor d. digital to analog converter

b. analog to digital converter The analog to digital converter is responsible for transforming the analog signal from the detectors into a digital form that may be used by the computer. Analog information is based on a scale, whereas digital information is in numerical form.

The component of the CT computer system responsible for the data processing of image reconstruction is the: a. pipeline memory b. array processor c. hard disk drive d. RAM microsprocessor

b. array processor The primary data processing component of the CT system is the array processor. It is responsible for receiving scan data from the host computer, performing all of the major processing of the CT image, and returning the reconstructed image to the storage memory of the host computer.

The reduction in intensity of an x-ray beam as it interacts with matter is called: a. scatter b. attenuation c. transmission d. luminescence

b. attenuation Attenuation is the reduction in intensity of an x-ray beam as interacts with matter. Several interactions are responsible for the attenuation of primary radiation, including Compton scatter hand photoelectric effect. Each of these interactions reduces the energy and/or number of the x-ray photons in the primary beam.

During date acquisition, a CT system may continuously adjust the mA relative to the measured image noise for a method of dose reduction referred to as: a. iterative reconstruction b. automatic tube current modulation c. attenuation correction d. Nyquist sampling

b. automatic tube current modulation Automatic tube current modulation programs adjust the mA throughout the acquisition to reduce patient radiation dose to a minimum. ATCM automatically alters the applied mA on the basis of the predetermined noise index that is acceptable for appropriate image quality.

The acquisition of a series of CT images at a single anatomic location over a set period is referred to as : a. ultrafast CT b. cine CT c. conventional CT d. temporal CT

b. cine CT Cine CT acquisition involves multiple axial scans obtained at a single anatomic level over a predetermined period. Clinical applications of cine CT acquisition include contrast bolus tracking, dynamic imaging of physiologic processes such as respiration, swallowing, and the cardiac cycle as well as for CT perfusion studies.

A test phantom containing water is scanned and five region of interest measurements are performed. The subsequent density measurements are compared and demonstrated a maximum deviation of less than 2HU. This quality assurance test was performed to evaluate the scanner's: a. linearity b. cross-field uniformity c. signal-to-noise ratio d. contrast resolution

b. cross-field uniformity The test described is used to evaluate the cross-field uniformity of the CT scanner. The ROI measurements are arranged with the one at the center of the image and the others rotated around the periphery. Each of the five ROI measurements taken from the water phantom should yield relatively the same CT number value. pg 109

The maximum number of simultaneous sections an MSCT system can acquire per gantry rotation is controlled by the number of: a. detector rows b. data channels c. focal spot d. X-ray tubes

b. data channels The number of data channels controls the number of sections that the scanner can simultaneously acquire with each gantry rotation. For example, a four slice MSCT System has 16 1.25 mm detectors in its array. However, with only four data channels, the maximum number of sections that may be reconstructed from each gantry rotation is four. The system combines transmission information from the individual detectors through it's four data channels, resulting in the acquisition of four sections per gantry rotation.

Which of the following would increase the noise apparent on a CT image? a. increase in section width b. decrease in aperture size c. increase in filtration d. decrease in matrix size

b. decrease in aperture size A decrease in the aperture size reduces the amount of x-ray photons exposing the tissue. This will cause a decrease in the signal-to-noise ratio. Keep in mind that the commenters used to adjust the size of the aperture absorb radiation as they restrict the beam.

Assuming no other compensating adjustments are made, increasing detector collimation _____ contrast resolution of a CT image. a. increases b. decreases c. does not change

b. decreases Increased detector collimation results in the reconstruction of thinner sections. As section width decreases, the photon flux for each pixel also decreases. Unless the mA setting is raised to compensate, an increase in noise occurs. Any increase in noise results in a decrease in contrast resolution. In fact, the major limiting factor for contrast resolution is noise. pg 106-107.

Which of the following units is used to quantify absorbed dose? a. roentgens b. grays c. sieverts d. curies

b. grays Absorbed does describes the amount of x-ray energy absorbed in a unit of mass. Is measured in grays.

The major disadvantage of the fan- or cone shaped x-ray beams used in modern CT units in comparison with the pencil-thin beams of older units is: a. increased transmission measurements b. greater patient radiation dose c. decreased transmission measurements d. excess tube wear

b. greater patient radiation dose The use of a fan or cone shaped x-ray beam during CT increases the total volume of tissue being radiated, Thereby increasing the amount of scatter radiation produced. This change results in a significantly higher patient radiation dose than pencil beam radiation used in older CT scanners.

Areas of a CT image that contain abrupt changes in tissue density are electronically represented by which of the following? a. positive CT numbers b. high spatial frequencies c. negative CT numbers d. low spatial frequencies

b. high spatial frequencies The contrast of a CT image is controlled by the spatial frequencies of the tissues within the section. Tissues of differing densities are represented electronically by different spatial frequencies. Adjacent tissues that greatly differ in density are represented by high spatial frequencies.

In the formula used to calculate the linear attenuation coefficient, the symbol "Io" identifies: a. Euler constant b. incident intensity c. absorber thickness d. transmitted intensity

b. incident intensity The Lambert-Beer law is used to calculate the attenuation coefficient of the volume of material. The symbol "Io" represents the intensity of the radiation incident upon the tissue being imaged. It is compared with the intensity of the radiation passing through and measured by the detector (I) during the calculation of the linear attenuation coefficient. "e" is Euler's constant and "x" is the object thickness.

Which of the following adjustments in technical factors will result in an increase in spatial resolution? a. increase in tube filament b. increase in sampling frequency c. increase in section width d. increase in display field of view

b. increase in sampling frequency The sampling frequency, or views per rotation, controls the volume of transmission data required for each gantry rotation. Very small objects exhibit high spatial frequencies that only systems with high sampling rates will be capable of resolving.

The average photon energy of the CT x-ray beam can be increased by: a. increasing mAs b. increasing filtration c. increasing collimation d. all of the above

b. increasing filtration Increases in filtration cause a greater amount of low energy x-ray photons to be absorbed, thereby increasing the average photon energy of the beam. Higher mAs values increase the intensity of the beam but do not affect the average photon energy.

The mathematical technique that involves the estimation of an unknown value from values on either side of it is known as: a. filtering b. interpolation c. convolution d. summation

b. interpolation Interpolation is a mathematical technique used in the reconstruction process of the spiral CT image. It involves the estimation of an unknown value for information above and below it. Convolution is the process of applying a mathematical filter to remove blurring from the reconstructed CT image.

Voxels with equal dimension along the x-, y-, and z-axes are referred to as: a. prospective b. isotropic c. retorspective d. anisotropic

b. isotropic Isotropic is used to describe voxels with equal dimensions along the x, y, and z-axes. The ability to acquire an isotropic data set is a distinct advantage of MDCT.

Which of the following types of image reconstruction was used in the first prototype CT scanner? a. convolution method b. iterative technique c. Fourier transform d. back projection

b. iterative technique

The broadening of the slice sensitivity profile inherent to helical CT acquisition is said to negatively affect the: a. in-plane spatial resolution b. longitudinal spatial resolution c. contrast resolution d. temporal resolution

b. longitudinal spatial resolution Longitudinal spatial resolution describes the degree to which patient movement during volumetric acquisition and can negatively affect image detail.

The loss of anatomic information between continuous sections due to inconsistent patient breathing is called: a. cupping artifact b. misregistration c. overshoot artifact d. out-of-field artifact

b. misregistration Misregistration is an artifact that occurs when a patient suspends respiration at different depths during consecutive scans. It results in the loss of anatomic information.

The fluctuation of CT numbers in the image of uniform, homogeneous material may occur because of: a. linearity b. noise c. aliasing c. partial volume effect

b. noise Noise appears on the CT image as an inaccuracy in CT number. The noise of a CT scanner may be measured by scanning a homogeneous object such as a water phantom. Fluctuations in CT number from pixel to pixel indicate the presence of noise.

The largest deterrent of a CT scanners contrast resolution is: a. patient motion b. noise c. hardware malfunction d. beam hardening

b. noise The largest enemy of contrast resolution is noise. Noise appears as a graininess that can obscure the outline and delineation of structures, thus limiting the scanner's ability to separate them.

Matrix size describes which of the following? a. aperture size used during the data acquisition b. number of pixels you use to display image c. relationship between the field of view and the algorithm d. the number of data channels available

b. number of pixels you use to display image Consisting of an arrangement of pixels in rows and columns, the matrix is used to organize the attenuation information from the anatomic section into a digital image. The size of the matrix is given as a number of pixels across multiplied by the number of pixels down.

Reformatted CT image planes that lie perpendicular to the original plane of acquisition may be described as: a. oblique b. orthogonal c. obtuse d. orthographic

b. orthogonal Orthogonal planes are at right angles to each other. Orthogonal reformations are perpendicular to the original plane of date acquisition.

The rate at which a quantity of x-radiation emitted from a CT tube passes through a unit area over a unit of time is called the: a. effective mAs b. photon flux c. constant mAs d. photon fluence

b. photon flux The rate at which a quantity of radiation (photon fluence) passes through a unit area over unit time is termed the photon flux. It may also be referred to as the fluence rate.

A CT image is displayed in a window with a level of 0 and a width of 500. Which of the following statements is correct? a. pixels with values between 0 HU and 500 HU will appear white b. pixels with values between -250 HU and +250 HU will be assigned shades of gray c. pixels with values greater than +500 HU will be black d. pixels with values less than 0 HU will appear white

b. pixels with values between -250 HU and +250 HU will be assigned shades of gray The width of a window determines the range of pixel values that will be assigned shades of gray around a given level. In this example, all pixels within the range of -250 to +250 HU will be assigned shades of gray. Pixels below -250 HU will appear black, and pixels above +250 HU will appear white.

Pixels representing tissues with average attenuation coefficients greater than that of water have which of the following types of values? a. extremely small b. positive c. negative d. low contrast

b. positive When calculated by a formula comparing the attenuation coefficient of tissue with that of water, materials whose coefficients are greater than that of water are assigned positive CT numbers.

A component of The DAS responsible for strengthening the signal that is emitted from a detector is termed the: a. analog to digital converter b. preamplifier c. photodiode d. photoconductor

b. preamplifier The weak electronic signal emitted from the detectors in response to the measurement of transmitted radiation must be strengthened before it is utilized for image reconstruction. The pre-amplifier is the component of the data acquisition system responsible for amplifying the detector's signal.

When one is choosing a window to display a CT image, the width defines the: a. midpoint of the range of pixels displayed b. range of CT numbers (pixels) to be displayed c. range of pixel values included in an ROI d. average CT number of the tissue of interest

b. range of CT numbers (pixels) to be displayed The width of a CT window controls the range of pixel values that are assigned a shade of gray. The width is centered on a level that is equal to the value of the tissue of interest.

The volume of a voxel maybe calculated by multiplying the pixel dimensions (mm^2) by the: a. display field of view b. section width c. pitch d. scan feel of view

b. section width As a geometric cuboid, The volume of the pixel maybe calculated as the product of the pixel area dimension in the section width. The pixel dimensions (d) must be first calculator by dividing the display field of view by the matrix size. The area of the pixel maven be found by squaring the value of d (mm). The unit of measurement for voxel volume is mm^3

Which of the following is not commonly used as a CT scintillation detector material? a. ceramic rare earth b. silver halide c. bismuth germanate d. cadmium tungstate

b. silver halide CT does not employ silver halide crystal as a detector material

MSCT systems typically employee which of the following types of detectors? a. tungsten ring b. solid-state scintillation c. charged coupled device (CCD) d. gas ionization

b. solid-state scintillation All modern MSCT systems utilize solid state detectors consisting primarily of a scintillating crystal material. Solid state detectors are preferred for MS CT because of their ability to accurately record instant x-ray energy from any angle. This flexibility is important when one considers the widened cone beam geometry inherent to MSCT systems.

Which of the following parameters or factors is improved by the selection of a smaller x-ray tube filament? a. scan time b. spatial resolution c. heat rating d. signal-to-noise ratio

b. spatial resolution Smaller focal spots improved geometric efficiency of the x-ray beam, leading to greater spatial resolution.

The ability of a CT scan or to image a small high-density object is controlled by the _____ of the scanner. a. contrast resolution b. spatial resolution c. sensitivity d. both a and c

b. spatial resolution The spatial resolution of a CT scanner controls its ability to image small structures.

The technique that provides the detail of the outer portions of a 3-D model while maintaining the typical CT detail of the lungs inside is called: a. maximum intensity projection (MIP) b. surface rendering c. summed projection d. voxel gradient

b. surface rendering Computer programs are capable of constructing 3-D models of anatomy with several different types of rendering techniques. A service rendered 3-D model provides excellent surface anatomy while maintaining the normal cross-sectional anatomy appearance of the volume inside the model.

When one is viewing a MPR image, each pixel represents: a. the maximum attenuation occurring within the voxel b. the average attenuation occurring within the voxel c. the minimum attenuation occurring within the voxel d. all attenuation occurring within the voxel above a set threshold value

b. the average attenuation occurring within the voxel A standard MPR image is 1 voxel thick, with the pixels facing the viewer each representing the average attenuation occurring within the represented voxels.

Which of the following statements comparing the efficiency of scintillation and gas ionization detectors is correct? a. both have approximately the same capture efficiency b. the scintillation detector has a higher capture efficiency c. unlike the scintillation detector, the gas ionization detector has a problem with afterglow d. gas ionization detectors have a higher conversion efficiency

b. the scintillation detector has a higher capture efficiency Because of its solid nature, the scintillation detector interacts with a higher percentage of incident x-ray photons, giving it a better capture efficiency.

For single-slice CT (SSCT) systems, which of the following statements regarding retrospective image reconstruction is FALSE? a. the algorithm, matrix size, and DFOV may all be changed b. the slice thickness and SFOV may be changed c. scan (raw) data must be available d. retrospective image reconstructions may be used to adjust the center of the image

b. the slice thickness and SFOV may be changed Retro recon uses scan or "raw" data to change the matrix, DFOV, center, and/or algorithm used for a CT image. The slice thickness and SFOV are specifically used for data acquisition and cannot be altered retrospectively with a single-slice CT (SSCT) system. The advent of a multislice CT (MSCT) technology has made retrospective adjustments in section width possible. The scan field of view (SFOV) remains an item that may not be altered retrospectively.

During a CT scan, each sample of ray sum measurements made by the data acquisition system is called a: a. signal b. view c. projection d. ray

b. view The term view is used to describe each data sample made by the DAS. A view contains transmission measurements made along multiple rays, each corresponding to a particular detector (ray sums). During the process of data acquisition, views are required by the DAS hundreds of times per second, according to the systems individual sampling rate.

An average CT number value for bone is: a. +100 HU b. +500 HU c. +1000 HU c. +3000 HU

c. +1000 HU An average CT number value for bone is about +1000 HU. This may vary widely with the density of the particular bone in question and with the beam quality of the CT scanner.

A CT system measures the average linear attenuation coefficient of a voxel of tissue to be 0.189. The attenuation coefficient of water for the scanner equals 0.181. The CT number assigned to the pixel representing this voxel of tissue equals: a. -456 HU b. -173 HU c. +44 HU d. +173 HU

c. +44 HU The CT number of a pixel may be calculated by subtracting the linear attenuation coefficient of water from the linear attenuation coefficient of the tissue within the voxel. This number is divided by the linear attenuation coefficient of water. The quotient is multiplied by contrast factor of 1000 to yield the value of the pixel in Hounsfield units. 0.189 - 0.181 = 0.008 0.008 / 0.181 = 0.044 0.044 x 1000 = +44

An average CT number value for blood is: a. -20 HU b. +10 HU c. +45 HU d. +100 HU

c. +45 HU An average range of CT numbers for blood is +42 to +58. The CT number of any material is based on how many factors, including the beam quality of a particular scanner.

The typical range for possible pixel values in a modern CT system is between: a. -512 HU and -512 HU b. -1000 HU and +1000 HU c. -1024 HU and +3071HU d. -4096 HU and +4096 HU

c. -1024 HU and +3071HU The typical range of Hounsfield scale CT numbers is -1024 HU and +3071HU. This range equals the 4096 different values possible when one is using a 12 bit system (2^12 = 4096).

A CT images reconstructed using a 512 x 512 matrix and the display field of view of 40 cm. What is the linear dimension of each pixel? a. 0.0015 mm b. 0.08 mm c. 0.78 mm d. 1.28 mm

c. 0.78 mm The dimension of a pixel maybe calculated by dividing the field of view by the matrix size. The DFOV of 40 cm must be first converted into 400 mm. This is then divided by 512 mm, for a pixel dimension of 0.78 mm. Keep in mind that the pixel is a square, two dimensional item and that the measurement of 0.78 mm corresponds to only one side.

Which of the following technical factors exhibits an effect upon longitudinal spatial resolution? 1. detector pitch 2. interpolation algorithm 3. display field of view a. 1 only b. 3 only c. 1 and 2 only d. 1 and 3 only

c. 1 and 2 only The factors that affect longitudinal spatial resolution and the SSP include the type of spiral interpolation algorithms inherent to the system and the selected detector pitch. DFOV has an effect on in-plane spatial resolution.

Which of the following technological advances has lead to the development of spiral/helical CT scanning? 1. slip-ring technology 2. electron beam technology 3. high-efficiency x-ray tubes a. 1 only b. 1 and 2 only c. 1 and 3 only d. 1, 2, and 3

c. 1 and 3 only

Collimation of the CT x-ray beam occurs: 1. at the x-ray tube, regulating slice thickness 2. just beyond the patient, focusing off-axis transmitted radiation 3. before the detectors, limiting the amount of scatter radiation absorbed a. 1 only b. 1 and 2 only c. 1 and 3 only d. 1, 2, and 3

c. 1 and 3 only Collimation of the x-ray beam in CT is accomplished by both pre-patient and pre-detectors restrict the field size, directly influencing the reconstructed section thickness. The pre-detector or post-patient detectors absorb scatter radiation before it contributes to the signal produced by the detector array.

Which of the following technical adjustments would decrease the quantum noise of the CT image? 1. increase mAs 2. decrease section width 3. increase section width a. 1 only b. 1 and 2 only c. 1 and 3 only d. 1, 2, and 3

c. 1 and 3 only The quantum noise of the CT image can be reduced by an increase in the number of x-ray photons reaching the detectors. this reduction may be accomplished by increasing mAs or section width (decrease collimation).

The types of detectors used in computed tomography are: 1) gas ionization 2) stimulable phosphor 3) scintillation a. 1 only b. 3 only c. 1 and 3 only d. 2 and 3 only

c. 1 and 3 only The types of detectors used in CT are scintillation and gas ionization detectors. Both operate by measuring the amount of transmitted radiation passed through the patient and transmitting this info to the computer for image reconstruction. Newer MDCT systems utilize solid-state scintillation detectors exclusively.

An acquisition is made on a 4-slice MSCT system with a detector array of 16 1.25-mm detector elements along the z-axis. With a selected beam width of 20 mm, what beam pitch results in the table moving 35 mm for each rotation of the gantry? a. 10 b. 1.5 c. 1.75 d. 2.0

c. 1.75 The beam pitch for a given acquisition is equal to the table feet per rotation divided by the total collimation. The total collimation for this acquisition is equal to the total number of sections (detectors) multiply by the detector dimension, or 16 multiplied by 1.25 mm. The beam pitch may be therefore be calculated by dividing 35 by 20, or 1.75.

A first-generation CT scanner consists of an x-ray tube and two detectors that translate across the patient's head while rotating in 1-degree increments for a total of: a. 45 degrees b. 90 degrees c. 180 degrees d. 360 degrees

c. 180 degrees First-generation CT scanners were based on a translate rotate principle. The x-ray tube and detectors would translate across the patient's head and then rotate 1 degree. This process would repeat in a semicircular fashion, for 180 degrees around the patient's head.

In a single slice CT (SSCT) system, which of the following technical parameters may be adjusted retrospectively? 1. section width 2. section increment 3. reconstruction algorithm a. 2 only b. 1 and 2 only c. 2 and 3 only d. 1, 2, and 3

c. 2 and 3 only Single slice helical geometry allows for volumetric data acquisition. The reconstruction of CT sections at any point along the scan volume is possible. The section width must be the same, however, and is controlled by the collimation (slice thickness) chosen prior to data acquisition. The advancement of multi slice CT (MSCT) allows for a retrospective reconstruction of sections at varying thicknesses.

Which of the following is considered an analytic form of image reconstruction? 1. iterative technique 2. Fourier reconstruction 3. filtered back-projection a. 1 only b. 1 and 2 only c. 2 and 3 only d. 1, 2, and 3

c. 2 and 3 only The analytic methods of CT image reconstruction include the filtered back-projection and the Fourier transform method. These techniques are called analytic because they utilize precise formulas for image reconstruction.

A quality control procedure determines that a low contrast resolution of the CT scanner is extremely poor. Likely causes are: 1. tube arching 2. increase electronic noise 3. decrease tube output a. 1 only b. 1 and 3 only c. 2 and 3 only d. 1, 2, and 3

c. 2 and 3 only The scanners reduced low contrast resolution is most likely the cause of increased noise. When the noise level of a CT image increases, the low contrast resolution decreases. Decreased patient radiation dose implies that the signal-to-noise ratio has decreased, thus raising the noise level of the image. The same situation would apply for an increase in electronic noise.

What is the maximum beam collimation for an MSCT system with 16 rows of 1.25 mm detectors? a. 2.5 mm b. 5.0 mm c. 20.0 mm d. 40.0 mm

c. 20.0 mm At a maximum, the MSCT cone beam may be collimated to a dimension equal to the entire multi row detector array. In the example, the total dimension of the detector array along the z-axis is 16 x 1.25 mm, or 20.0 mm.

Which of the following acquisitions may be characterized as contiguous? a. 2.5 mm sections reconstructed every 1.25 mm b. 5.0 mm sections reconstructed every 7.5 mm c. 3.75 mm sections reconstructed every 3.75 mm d. 20 second cine acquisition with 1.25 mm sections

c. 3.75 mm sections reconstructed every 3.75 mm Contiguous images are those acquired with equal section thickness and internal. For example, 3.75-mm sections reconstructed every 3.75 mm completely cover a given volume of tissue with no unmeasured tissue.

An acquisition is made on a 64-slice MSCT system in which each detector element has a z-axis dimension of 0.625 mm. With a selected beam width of 40 mm and the beam pitch set at 1.50, how many mm will the table move with each rotation of the gantry? a. 20 mm b. 40 mm c. 60 mm d. 80 mm

c. 60 mm The beam pitch for a given acquisition is equal to the table feed per rotation divided by the total collimation. The total collimation for this acquisition is equal to the total number of sections (detectors) multiplied by the detector dimension, or 64 x 0.625 mm. The table feed per rotation can be calculated as the produce of the total collimation and the beam pitch, or 40mm x 1.5, and equals 60 mm for the example provided.

CT generally employs kVp levels of _____ to _____. a. 120-140 b. 100-140 c. 80-140 d. 90-110

c. 80-140 pg. 76 (mosby)

In 1979, the scientists _____ and _____ shared the Nobel prize for their research in CT. a. Hounsfield and Watson b. Hounsfield and Olendorf c. Hounsfield and Cormack d. Hounsfield and Radon

c. Hounsfield and Cormack

A CT image is formed in part by projecting back all of the attenuation values recorded during data acquisition onto a: a. pixel b. voxel c. matrix d. reformat

c. Matrix The back projection method of image reconstruction involves the acquisition of attenuation values, which are then projected back onto a matrix for subsequent display.

CT image is displayed in a window with a level of +200 and a width of 1000. Which of the following statements is correct? a. Pixels with values between +200 HU and -1000 HU will appear white b. Pixels with values greater than +200 HU will appear black c. Pixels with valleys between -300 HU and +700 HU will be assigned shades of gray d. Pixels with values between -1200 HU and +1200 HU will be assigned shades of gray

c. Pixels with valleys between -300 HU and +700 HU will be assigned shades of gray The width of the window determines the range of pixel values that will be assigned the shade of gray around a given level. In this example, all pixels within the range of -302 to +700 HU Will be assigned shades of gray. Pixels below -300 HU will appear black, and pixels above +700 HU will appear white. This calculation is performed by dividing the width in half (1000/2 = 500) and subtracting and adding this value to the level (+200 HU + 500 = +700 HU, +200 HU - 500 = -300 HU).

Which of the following is not used to archive CT images? a. 3.5" floppy disc b. magnetic tape c. VHS tape d. magnetic optical disc

c. VHS tape

Writable CD and DVD media are optical discs that may be described as: a. read-only b. lossy c. WORM d. lossless

c. WORM Write-once, read-many (WORM) describe certain CD and DVD media capable of storing data only once. Distorted images can be read multiple times, but once recorded on, the media may not be reused.

In an MSCT system, the detector array is composed of multiple rows of individual detector elements along the: a. X axis b. Y axis c. Z axis d. entire circumference of the gantry

c. Z axis A multi slice CT system utilizes a third-generation curvilinear detector array with multiple rows of individual detector elements segmented along the longitudinal axis, or z-axis.

When an operator reduces the scan field of view for particular body part which the following technical changes occurs? a. the displayed image appears larger b. spatial resolution increases c. a smaller number of detectors are activated d. the displayed image appears smaller

c. a smaller number of detectors are activated The SFOV size is determined by the number of detectors activated along the inplane (X and Y axes) during data acquisition.

As a solid-state CT detector measures transmitted radiation, it emits a proportional response in the form of a(n): a. digital signal b. quantity of gas ions c. analog signal d. modulation transfer function

c. analog signal The detector emits an electronic analog signal in proportionate response to the transmitted radiation it absorbs. As the detector array rotates around the patient, the signal varies according to the measured x-ray energy flux.

When the system accounts for the properties of each ray sum and correlates it to the position of the ray, it is called a/an__________. a. filter function b. view c. attenuation profile d. back projection

c. attenuation profile An attenuation profile is created for each view of the scan. The information from all of the profiles is projected onto a matrix. This process of converting the data from the attenuation profile to a matrix is know as back projection (compiles the info from all of the attenuation profiles to create an image). Pg 26 (CT for techs)

The CT "window" controls the _____ of the CT image as it appears to the viewer. a. density and detail b. spatial and contrast resolution c. contrast and brightness d. attenuation coefficient and Hounsfield value

c. contrast and brightness The operator may control the contrast and brightness of the CT image by adjusting the window setting. As a form of gray-scale mapping, the window determines the pixels assigned shades of gray on the basis of their CT numbers.

Which of the following actions would serve to magnify the CT image on the display monitor? a. decrease matrix size b. increased scan field of view c. decrease display field of view d. increase display field of view

c. decrease display field of view Referred to as targeting, decreasing the display field of view causes in increase in the image size on the monitor. The DFOV controls the amount of scanned information to be displayed on the matrix. If a small portion of information is to be displayed on the entire matrix, it will appear larger to the viewer.

Which of the following is capable of causing an edge gradient artifact? a. detector malfunction b. involuntary patient motion c. dense bone d. tube arcing

c. dense bone The edge gradient effect occurs in areas of abrupt change in density, which are represented by high spatial frequency signal. The computer may have difficulty interpreting this type of rapidly changing information, and streaks may appear as the result. The interface between dense bone and soft tissue is a common site for the edge gradient effect to occur.

The component of the computer that changes the signal from the computer memory back to a format so that the image can be displayed on the monitor. a. DAS b. analog to digital converter c. digital to analog converter d. array processor

c. digital to analog converter Because the monitors used in CT work with analog signals, it is necessary to convert the digital signal from the computer's memory back to an analog format. The DAC accomplishes this task. Pg 31 (CT for techs)

In the binary number system, a byte is a series of _____ bits of information. a. two b. four c. eight d. sixteen

c. eight A byte is a series of eight bits. Bits and bytes are part of the binary language used by computers to process information.

Which of the following statements about collimation of the CT x-ray beam is false? a. collimation of the x-ray beam occurs both before and after the patient b. collimation of the beam occurs in the z-axis, thus affecting slice thickness c. increases in collimation increase the intensity of the primary beam d. collimation of the CT x-ray beam is used to limit the detection of scatter radiation

c. increases in collimation increase the intensity of the primary beam Collimation occurs as lead shutters close down upon the beam, limiting its projected area. Constructed of lead, the collimator shutters absorbed portions of the primary beam, thereby reducing its intensity.

The mathematical technique allowing the reconstruction of motion-free images from helically acquired CT data is called: a. convolution b. radon transformation c. interoplation d. Fourier reconstruction

c. interoplation Interpolation is the mathematical process whereby data from tube rotations just above and just below a given slice position are used for image reconstruction. Interpolation allows for the reconstruction of a thin, motion free image from a volumetric data set acquired from a moving patient. The process of applying a mathematical filter to remove blurring from the reconstructed CT image is termed convolution.

Which of the following mathematical techniques is used exclusively for image reconstruction with helical CT scanners? a. back projection b. convolution c. interpolation d. iterative techniques

c. interpolation Interpolation is a mathematical technique used in helical CT. With interpolation, approximations of information above and below a section level are made to overcome the fact that a complete revolution of tube and detectors was not performed for each image.

Which of the following factors may affect the attenuation of an object during CT data acquisition? a. beam pitch b. mA c. kVp d. algorithm

c. kVp The degree of attenuation depends on multiple factors, including the x-ray energy and the atomic structure and density of the exposed tissue.

Which of the following factors does not affect the dimensions of a voxel? a. slice thickness b. matrix size c. kernal d. display field of view

c. kernal The dimensions of a voxel are determined by multiplying the pixel dimensions by the slice thickness. The dimensions of a pixel are directly controlled by the matrix size and the field of view.

A CT image is formed in part by projecting back all of the attenuation values recorded during data acquisition onto a: a. pixel b. voxel c. matrix d. reformat

c. matrix The back-projection method of image reconstruction involves the acquisition of attenuation values, which are then projected back into a matrix for subsequent display.

The process of displaying CT images in a different orientation from the one used in the original reconstruction process is called: a. retrospective reconstruction b. prospective reconstruction c. multiplanar reformation d. multisegment reconstruction

c. multiplanar reformation Multiplanar reformation describes the process of displaying CT images in a different orientation from the one used in the original reconstruction process. Unlike retrospective reconstruction, reformation does not change the makeup of the image voxels. What Information merely alters the viewing perspective of the image to a different anatomic plane.

Pixels whose average attenuation coefficients are less than the coefficient of water have which of the following types of CT number values? a. extremely large b. high positive c. negative d. high contrast

c. negative When a formula comparing the attenuation coefficient of tissue with out of water is used, materials who's coefficients are less than that of water are assigned negative CT numbers.

Bow-tie filters are employed in the CT x-ray system to reduce: a. image noise b. low spatial frequency signal c. patient radiation dose d. high spatial frequency signal

c. patient radiation dose Bow-tie filters, which are thicker at the ends than in the middle, help shape the beams to reduce patient radiation exposure. Because most body parts are circular or cylindrical, less radiation is necessary at the periphery then in the center. Bow-tie filters reduce beam intensity toward the outer margins, resulting in the reduction in patient radiation dose.

Which of the following 3-D imaging techniques is utilized to provide "fly-through" images during CT colonography? a. orthographic volume rendering b. shaded surface display (SSD) c. perspective volume rendering d. minimum intensity projection (min-IP)

c. perspective volume rendering Perspective volume rendering provides one the viewpoint of being within the lumen of the object, similar to an end of scope view. Also referred to as immersive rendering, this type of 3D reconstruction is commonly used during CT colongraphy and CT bronchography.

Which of the following is a solid-state device used to record the light flashes given off by a scintillation crystal? a. photomultiplier tube b. anode c. photodiode d. input phosphor

c. photodiode Scintillation crystals are used in cooperation with photodiodes in a scintillation-type CT detector. The photodiode is a solid-state device that absorbs the light flashes given off by the crystal. The photodiode then emits an electrical signal in response to this light. A photomultiplier tube detects weak light signals and amplifies the electrons generated by a photocathode exposed to a photon flux.

The measurement of CT system performance through quality testing procedures and evaluation of the test results is referred to as: a. calibration b. uniformity c. quality assurance d. quality control

c. quality assurance Quality assurance refers to the measurement of the scanners performance through quality testing procedures and evaluation of the test results. Quality control refers to the implementation of corrective actions to improve any identification performance inadequacies of the CT system found throughout quality assurance procedures. pg 109

Which of the following types of the image noise can be most easily reduced by the CT technologist? a. electronic noise b. artifactual noise c. quantum noise d. detector noise

c. quantum noise The quantum or statistical noise of a CT image can be reduced by increasing the number of x-ray photons absorbed by the detectors for each voxel of tissue. This can be accomplished with increases in technique (mAs) in with increases the pixel and voxel dimensions.

The most effective method of reducing involuntary motion on the CT image is: a. immobilization b. thorough explanation of the exam to the patient c. reduced scan times d. physical restraint

c. reduced scan times Involuntary motion is beyond control of the patient and includes peristalsis, cardiac contraction, and tremors. The most effective method of reducing involuntary motion on the CT scan is reduction of scan times. Many scanners offer segmenting or "half-scan" options whereby images may be reconstructed after a partial revolution of the tube-detector system. In the case of digestive involuntary motion, glucagon administration is an additional option.

An image that is reconstructed a second time with some change in technical factor is said to be: a. reiterated b. post processed c. retrospective d. reformatted

c. retrospective Retrospective reconstruction occurs when an image is reconstructed a second time with an adjustment in technical factors. The scan data or "raw" Data are used to reconstruct the image with a different display matrix, display field of view, algorithm, and so on.

Ring artifacts on the CT image are associated with which of the following tube detector geometries? a. rotate-nutate b. rotate - stationary c. rotate - rotate d. translate - rotate

c. rotate - rotate Ring artifacts are associated with use of third-generation CT scanners. Both the x-ray tube and the detector array rotate around the patient with third-generation scanners. A malfunctioning detector or series of detectors in a third-generation CT scanner causes a ring artifact to appear on the image as a result of the rotational nature of the detector array. Rotate-stationary tubes are fourth-generation.

Fourth-generation CT scanners use a _____ tube-detector configuration. a. rotate-translate b. electron beam-stationary c. rotate-stationary d. rotate-rotate

c. rotate - stationary Fourth-generation CT scanners operate with a rotating x-ray tube and a stationary ring of detectors. Some fourth-generation scanners also use the rotating x-ray tube with the nutating detector ring.

The distance between the centers of two adjacent reconstructed CT images is termed the: a. section width b. interpolation degree c. section interval d. sampling rate

c. section interval Section interval describes the spacing between two adjacent CT images. It is measured as a distance between the center of one section in the center of the adjacent section.

CT images that have been reconstructed from a portion of the data acquisition process in hopes of reducing patient motion artifacts are called: a. dynamic images b. subtraction images c. segmented images d. filtered images

c. segmented images CT images are usually constructed from transmission data acquired during a 360° rotation of the x-ray tube. A CT image can also be constructed from the portion of the data acquisition. For example, if a patient moves during the last third of a two second scan, an image can be constructed from the first 240° of tube rotation, yielding an image free of motion. This process, referred to as segmentation, has a software capability of many modern CT scanners.

Which of the following units is used to quantify effective dose? a. roentgens b. grays c. sieverts d. curies

c. sieverts Effective dose approximates the relative risk from exposure to ionizing radiation in this based on the radio sensitivity of a given tissue type. It is measured in sieverts (Sv).

The continuous gantry rotation required by helical CT acquisition is made possible by the application of: a. electromagnetic bushings b. titanium bearings c. slip-rings d. oil-cooled couplings

c. slip-rings Eliminating the need for cables, slip-ring technology allows for continuous gantry rotation by utilizing a system of contact brushes that supply electricity to power the system, enabling the passage of transmission data to the computer system.

The ability of an MSCT system to freeze motion and provide an image free of blurring is called: a. in-plane spatial resolution b. longitudinal spatial resolution c. temporal resolution d. contrast resolution

c. temporal resolution These stop motion capability of The CT system is referred to as temporal resolution. Temporal resolution quantifies the CT system's ability to freeze motion and provide an image free of blurring.

The term beam hardening is used to describe which of the following physical phenomena? a. the decrease in average photon energy of the heterogeneous x-ray beam b. the increase in average photon energy of the homogeneous x-ray beam c. the increasing average photon energy of the heterogeneous x-ray beam d. the decrease in the average photon energy of the homogeneous x-ray beam

c. the increasing average photon energy of the heterogeneous x-ray beam Being hardening occurs as a x-ray beam traverses the patient. The x-ray beam used in CT is composed of x-ray photons with numerous different energies. The heterogeneous beam undergoes an increase in average photon energy as it passes through the patient in the lower energy photons are absorbed.

When one is viewing a minimum intensity projection (min-IP) image, each pixel represents: a. the maximum attenuation occurring within the voxel b. the average attenuation occurring within the voxel c. the minimum attenuation occurring within the voxel d. all attenuation occurring within the voxel above set threshold value

c. the minimum attenuation occurring within the voxel MIn-IP Image displays the minimum pixel value along each ray to the viewer. As the viewer faces a min-IP image, each pixel represents the minimum attenuation that occurs in the associated voxel. Min-IP reformations are used primarily during the evaluation of the binary tree, colon, lungs, and trachea.

MSCT systems typically employ which of the following detector geometries? a. first generation b. second generation c. third generation d. fourth generation

c. third generation All MSCT systems utilize third-generation geometry and solid-state scintillation detectors.

The information included during the 3D reconstruction of a CT scan is controlled by the: a. algorithm b. window setting c. threshold setting d. gray-scale map

c. threshold setting The threshold setting is used to include and exclude information during the 3D reconstruction of a CT scan. For example, a high threshold (+150 HU) may be set to produce a 3D model of a bony structure. This threshold eliminates any density value below +150 HU from the data set. The reconstructed 3D model would include only bone tissue or any other substance with an HU value above +150.

In multi slice CT, the beam pitch is equal to the table feed per rotation divided by the: a. number of detectors b. pre-patient collimation c. total collimation d. number of channels

c. total collimation The beam pitch, which is unique to multi slice CT, is equal to the table feed per rotation divided by the total collimation. The total collimation is equal to the combined thickness of all the sections are simultaneously acquired with each gantry rotation.

First generation CT scanners use the method of data acquisition based on a _____ principle. a. multiplanar b. rotation only c. translate-rotate d. transaxial

c. translate-rotate The first generation CT scanner required data through a process based on the principle of tube translation and rotation around the patients head.

The intensity of the x-ray beam after it passes through an object to a detector is called the: a. incident intensity b. ray c. transmitted intensity d. primary beam

c. transmitted intensity The transmitted intensity is the amount of energy that passes through a patient onto a detector.

The ability of a CT system to maintain consistent HU values across the entire image of a homogeneous object is termed: a. linearity b. calibration c. uniformity d. contrast resolution

c. uniformity The uniformity of a CT system describes its ability to maintain relatively consistent CT values across the entire image of an object of equal density, such as a water-filled phantom.

The Hounsfield value of the pixel is directly related to which of the following? a. window width b. field of view size c. μ of HO d. window level

c. μ of HO The value of the pixel in Hounsfield units is calculated through a comparison of the linear attenuation coefficient (μ) of a voxel of tissue to that of water.

A CT scanner measures the linear attenuation coefficient of the voxel of tissue as 0.40. The linear attenuation coefficient of water for the scanner equals 0.20. The CT number assigned to the pixel representing this voxtel of tissue equals: a. -1000 HU b. 0 HU c. 1 HU d. +1000 HU

d. +1000 HU CT number of the pixel maybe calculated by subtracting the linear attenuation coefficient of water from the linear attenuation coefficient of the tissue within the voxel. This number is divided by the linear attenuation coefficient of water. The quotient is multiplied by contrast factor of 1000 to yield the value of the pixel in Hounsfield units.

At the range of photon energy that is employed during most CT examinations, water exhibits an approximate attenuation coefficient value of _____. a. 0.0007 b. 0.051 c. 0.185 d. 0.206

d. 0.206

Which of the following sets of technical factors would yield an isotropic data set when an MDCT image is reconstructed using a 512 x 512 matrix? a. 5.0 mm sections, 17 cm DFOV b. 3.0 mm sections, 25 mm DFOV c. 1.25 mm sections, 48 mm DFOV d. 0.625 mm sections, 32 mm DFOV

d. 0.625 mm sections, 32 mm DFOV Thin-section reconstruction using the smallest display field of view possible results in the smallest pixel and voxel dimensions. Voxels with equal dimensions along the X, Y, and Z axes are described as isotropic.

Which of the following reconstruction algorithms are used to overcome the divergent nature of MSCT beams and the lack of detector information at the beam's periphery? 1. Feldkam-David-Kress 2. Lambert-Beer 3. advanced single-slice rebinning a. 1 only b. 2 only c. 1 and 2 only d. 1 and 3 only

d. 1 and 3 only The Lambert-Beer equation is used to calculate the linear attenuation coefficient of an object is valid only for homogeneous pencil-beam radiation sources such as the first-generation CT system. pg 89-90

When choosing a scan field of view (SFOV), the CT technologist is controlling the: 1. diameter of data acquisition 2. Number of activated detectors within the array 3. correction factors for the specific area of anatomic interest a. 2 only b. 1 and 2 only c. 1 and 3 only d. 1, 2, and 3

d. 1, 2, and 3 Each CT scanner has several choices for SFOV. The choice made by the technologist activates a certain percentage of the detector array so that information is acquired from a circular portion of the anatomic section. Built into the SFOV selection are additional correction factors used to process different types of tissue. For example, a CT scanner may have a specific selection for scans of the head, which attempts to limit the artifact occurring at the delineation of bone from brain tissue.

Which of the following components of CT image quality may be controlled by the technologist? 1. spatial resolution 2. contrast resolution 3. noise a. 2 only b. 3 only c. 1 and 3 d. 1, 2, and 3

d. 1, 2, and 3 Each component of the CT image quality may be affected- either positively or negatively-by the CT technologist. Spatial resolution is affected by geometric factors such as focal spot size and slice thickness. Contrast resolution maybe altered by slice thickness, algorithm selection, and noise. The noise level of an image is dependent upon several factors, including patient dose and pixel dimensions. Each of these factors must be considered by the CT technologist in an effort to provide high-quality CT studies.

The mathematical manipulations acquired during the reconstruction of the CT image is accomplished using a(n): 1. algorithm 2. kernal 3. mathematical filter function a. 1 only b. 1 and 2 only c. 1 and 3 only d. 1, 2, and 3

d. 1, 2, and 3 Terms algorithm, kernel, and mathematical filter function may all be used interchangeably in describing the mathematical process used for complex calculations required during CT image reconstruction.

The data acquisition system (DAS) of a CT scanner is responsible for: 1. measuring transmitted intensity 2. conversion of the transmission and data into a digital signal 3. sending the digital information to the computer for processing a. 1 only b. 2 only c. 1 and 3 only d. 1, 2, and 3

d. 1, 2, and 3 The DAS of a CT scanner consists of the detector array, the analog-to-digital converter, and a transmission device used to send the converted digital information to the computer for image reconstruction.

Which of the following technical factors exhibit an effect upon longitudinal spatial resolution? 1. detector collimation 2. detector pitch 3. interpolation algorithm a. 1 only b. 2 only c. 1 and 2 only d. 1, 2, and 3

d. 1, 2, and 3 The factors that affect longitudinal spatial resolution and the SSP include the type of spiral interpolation algorithm inherent to the system and the selected detector pitch. Thin-section image reconstruction reduces the partial volume effect, reduces or completely eliminates broadening of the SSP, and improves longitudinal spatial resolution. pg 106

Which of the following mathematical functions may be used to quantify the spatial resolution of a CT scanner? 1. PSF 2. MTF 3. LSF a. 1 only b. 1 and 3 only c. 2 and 3 only d. 1, 2, and 3

d. 1, 2, and 3 The linespread function, point spread function, and modulation transfer function are partly used to quantify the spatial resolution of a CT scanner. The LSF measures the ability of the CT scanner to clearly image the edge of a line. The PSF does the same for extremely small, point like structures. The MTF examines the fidelity of the spatial frequency as it represents tissues with the varying densities. The MTF is derived from the measurement of the LSF and PSF.

Which of the following technical adjustments may be employed to improve the temporal resolution of an MDCT system? 1. decrease in section width 2. image segmentation 3. physiologic gating a. 1 only b. 3 only c. 1 and 2 only d. 2 and 3 only

d. 2 and 3 only Technical adjustments to improve the temporal resolution of the CT system may help reduce involuntary motion artifacts. Reductions in scan time (gantry rotation), image segmentation, physiologic gating can be used to reduce the effects of involuntary motion.

Which of the following terms is commonly used to describe a CT scanner's ability to differentiate objects with similar linear attenuation coefficients? 1. spatial resolution 2. sensitivity 3. contrast resolution a. 1 only b. 3 only c. 1 and 2 only d. 2 and 3 only

d. 2 and 3 only The ability of a CT scanner to accurately image objects with similar density is termed contrast resolution or sensitivity. A CT scan with low contrast resolution or low sensitivity has difficulty separating tissues whose linear attenuation coefficients are nearly equal.

Which of the following statements concerning the translate-rotate mode of CT data acquisition is correct? 1. 360 Circular detector arrays are used 2. data are collected only during translation 3. was used in first- and second-generation CT scanners a. 2 only b. 3 only c. 1 and 2 only d. 2 and 3 only

d. 2 and 3 only The first-and second-generation CT scanners used a translate-rotate method of data acquisition. The x-ray tube and detectors translated across the patients head, recording transmission measurements. The entire system then rotated 1°. This process of translation-rotation then repeated itself for a total of 180°. First- and second-generation CT scanners used anywhere from 2 to 30 detectors.

Which of the following technical factors is/are involved in the determination of section width for MDCT images? 1. scan field of view 2. beam collimation 3. detector configuration a. 2 only b. 1 and 2 only c. 1 and 3 only d. 2 and 3 only

d. 2 and 3 only The thickness of the reconstructed section is controlled primarily by the detector configuration in an MDCT system. Beam collimation also exerts an indirect effect on section width by controlling the portion of the detector array exposed to transmitted radiation. The SFOV determines the size of the fan beam (x and y axis), which, in turn, determines the number of detector elements that collect data. Pg 17 (CT for techs)

An accurate, modern CT scanner possesses a spatial resolution up to: a. 10 lp/mm b. 25 lp/mm c. 10 lp/cm d. 25 lp/cm

d. 25 lp/cm The limiting resolution of a modern CT scanner is approximately 25 lp/cm. This resolution varies greatly with scan factors and is considerably less than that of projection radiography. (2.5 lp/mm)

The maximum beam collimation for an MDCT system with the detector array of 64 detectors, each 0.625 mm wide is: a. 1.25mm b. 5.0 mm c. 10.00mm d. 40.00mm

d. 40.00mm At a maximum, the MSCT cone beam maybe collimated to a dimension equal to the entire multirow detector array. In the example, the total dimension of the detector array along the z-axis is 40.00 mm, or 64 x 0.625mm.

In a modern CT system, the total number of possible Hounsfield values that may be assigned to anyone pixel is approximately: a. 512 b. 1024 c. 2048 d. 4096

d. 4096 The CT computer system is typically capable of displaying 12 bits of data per pixel and is therefore able to display any of up to 4096 Hounsfield values for each pixel (2^12 = 4096).

What matrix size was used to reconstruct an image with the display characters DFOV 25cm and pixel area 0.25mm x 0.25mm a. 80 x 80 pixels b. 256 x 256 pixels c. 320 x 320 pixels d. 512 x 512 pixels

d. 512 x 512 pixels The matrix size use to reconstruct an image can be calculated by dividing the display field of view by the pixel dimensions. A pixel whose area is 0.25mm x 0.25mm has a linear dimension of 0.5mm. Dividing the DFOV of 250mm by the pixel dimensions 0.5 mm gives a matrix size of approximately 512 x 512 pixels.

Power output for modern MDCT x-ray tubes has an approximate range of: a. 3-5 kW b. 15-20 kW c. 40-48 kW d. 60-100 kW

d. 60-100 kW The power output of modern MDCT systems is vendor specific, within a typical range of 60 to 100 kW.

Digital CT images are networked and archived in accordance with a standardized computer protocol identified by which of the following abbreviations? a. HIPAA b. HTTP c. TCP/IP d. DICOM

d. DICOM The Digital Imaging and Communications in Medicine (DICOM) standard confirms the process of recording, storing, printing, and transmitting medical image data.

During which of the following CT examinations is a miss registration artifact most likely to occur? a. brain b. pelvis c. neck d. abdomen

d. abdomen Misregistration is a loss of anatomic information that occurs when a patient suspends respiration at different depths during consecutive scans. It occurs only during CT examinations and with suspended respiration of the patient is necessary. The ability of MDCT systems to volumetricly acquired entire anatomic areas in a single, short breath hold has greatly reduced the occurrence of misregistration artifact.

Which of the following is considered an equipment-induced CT image artifact? a. step artifact b. metallic artifact c. cupping artifact d. aliasing artifact

d. aliasing artifact Aliasing is a type of streak artifact caused when an insufficient number of views, data samples, are obtained during data acquisition. It is typically due to the technical failure of the CT system.

The geometric efficiency of a CT detector is influenced primarily by the: a. atomic number of the detector material b. size of the detector element c. size of the tube filament d. amount of interspace material between detectors

d. amount of interspace material between detectors The geometric efficiency have a CT detector is influenced primarily by the amount of interspace material necessary between adjacent detectors. Although valuable in limiting interference (cross talk) between adjacent detectors, the interspace material reduces geometric efficiency by absorbing transmitted x-ray energy.

The component of the CT scanner responsible for the mathematical calculations of the image reconstruction process is the: a. DAS b. analog to digital converter c. digital to analog converter d. array processor

d. array processor The array processor is a specialized component of the CT computer system. It is capable of performing the massive calculations required for CT image reconstruction.

Region of interest measurement placed over the portion of a CT image provides which of the following? a. distance (mm) b. diameter (mm) c. linear attenuation coefficient d. average CT number (HU)

d. average CT number (HU) The region of interest measurement provides a quantitative analysis of the Hounsfield values for specific anatomic area. System software calculates the average CT number in HUs within the defined ROI. The average ROI measurement provides information regarding tissue characteristics that maybe helpful for for clinical diagnosis.

The Feldkamp-Davis-Kress (FDK) algorithm may be applied to MSCT acquisition data to overcome image artifacts from: a. patient motion b. beam hardening c. partial volume averaging d. beam divergence

d. beam divergence The beam divergence inherent to the cone beam geometry of MSCT can cause difficulties during the image reconstruction process. Specific cone beam algorithms such as the Feldkamp-Davis-Kress (FDK) and advanced single slice rebinning (ASSR) algorithms attempt to overcome the tissue resulting from the convergent path of the x-ray beam from tube to the widened detector array.

Phantom measurement of the uniformity of a CT system is performed primarily to assess the negative effects of: a. partial volume averaging b. noise c. detector drift d. beam hardening

d. beam hardening The uniformity of a CT system describes its ability to maintain relatively consistent CT values across the entire image of an object of equal density, such as a water filled phantom. Beam hardening occurs as a low energy x-ray photons are absorbed when the beam passes through the patient. The average photon energy of the beam increases along the path may result in a loss of system uniformity.

Cupping most commonly occur in the: a. chest b. abdomen c. pelvis d. brain

d. brain Cupping artifacts occur when the periphery of an image is much higher intensity than the center. This difference in density causes beam hardening to negatively affect the accuracy of the image. The dense bone that surrounds the soft tissue of the brain is a common sight for copying artifacts to occur.

Which of the following image artifacts is unique to multislice CT (MSCT) systems? a. step artifact b. alaising artifact c. out-of-field artifact d. cone beam artifact

d. cone beam artifact Cone beam artifacts are unique to the cone beam geometry of MDCT systems and typically manifest much like partial volume artifacts.

Which of the following best describes the shape of the x-ray beam utilized in MDCT? a. pencil beam b. fan shaped c. electron beam d. cone shaped

d. cone shaped Multislice CT utlilizes a cone-shaped beam that is incident upon an expanded array of detectors. The detector array consists of multiple rows along the z-axis and requires a cone-shaped beam to measure incident radiation

During CT x-ray exposure, the product of the selected mA setting and the scan time is called the: a. effective mAs b. peak mAs c. absorbed mAs d. constant mAs

d. constant mAs The mA setting in coordination with the scanned time (seconds) gives the constant mAs for a CT acquisition.

Which of the following will serve to decrease the noise of a CT image? a. decreasing in dose b. decrease in slice thickness c. increase in matrix size d. decrease in matrix size

d. decrease in matrix size A decrease in matrix size will cause a subsequent increase in pixel dimensions. This larger size causes an increase in the number of x-rays passing through each pixel, thereby increasing the signal-to-noise ratio.

_____ describes the calculated mAs value per acquired slice. It equals mAs divided by pitch. a. photon flux b. photon fluence c. constant mAs d. effective mAs

d. effective mAs The main controlling factor of effective mAs is table speed. The pitch selected for a given acquisition determines the speed at which the patient translates through the gantry for each tube revolution. As pitch increases, the mAs applied to each acquired slice decreases. pg 76 Constant mAs is the product of the selected mA setting and the scan time.

The full width at half maximum of a slice sensitivity profile diagram indicates the: a. limiting resolution b. signal-to-noise ratio c. dose profile d. effective section width

d. effective section width The effective section width is defined as the full width at half maximum of the SSP. It is measured by examining the slice sensitivity profile at half of its maximum height.

To maintain security and patient privacy when medical images are transmitted across the network, the images maybe coded in the form of decipherable only by software at the destination location. This process is referred to as: a. write-once, read-many (WORM) b. hypertext transfer protocol (HTTP) c. teleradiology d. encryption

d. encryption Digital encryption is used to encode data that is transferred across the network into a form decipherable only buy software at the destination location

Which of the following mathematical reconstruction methods is used by most modern CT scanners? a. back-projection b. iterative methods c. Fourier transform d. filtered back-projection

d. filtered back projection The filtered back projection, or convolution, method of imagery reconstruction is used by most modern CT scanners.

During CT X-ray reproduction, the electromagnetic steering of the electron beam from the cathode to two alternating targets is referred to as: a. prospective gating b. ultra fast CT c. twin CAT d. flying focal spot

d. flying focal spot The oversampling technology known as flying focal spot involves electromagnetic steering of the electron beam emitted from the cathode. The beam of electrons is directed toward two separate locations on the rotating anode, resulting in two sources of x-radiation. As the tube rotates around the patient, the number of data samples is essentially doubled because of the electronic switching between the two focal spots, improving the systems temporal and spatial resolution.

The device constructed to house the x-ray tube and data acquisition system (DAS) for a CT scanner is called the: a. CPU b. generator c. array processor d. gantry

d. gantry The x-ray tube and data acquisition system are housed within the gantry of the CT scanner. The gantry aperture is a circular opening through which the patient moves during scanning.

Statistical noise appears as _____ on a CT image. a. decreased contrast b. increased brightness c. concentric circles d. graininess

d. graininess Statistical noise is a term that may be used for quantum noise, or mottle. Caused by an insufficient number of photons being detected, this type of noise appears as graininess on the CT image.

Which of the following technical changes may increase the partial volume affect present on a spiral CT scan? a. decrease in pitch b. decreasing in section width c. increase in matrix size d. increase in pitch

d. increase in pitch When the pitch is increased during a spiral CT scan, either the section width for the table speed has been increased. The effects of increased section width on the partial volume affect are obvious. The increased table speed also plays a role in increasing partial volume averaging. Because the table move through the gantry at a faster rate, each rotation of the tube and detectors is responsible for recording more information. This causes a broadening of the sec. sensitivity profile (SSP), which manifests as partial volume averaging.

Which of the following would increase the signal-to-noise ratio of a CT image? a. decreased aperture size b. decreased mAs c. increase filtration d. increased aperture size

d. increased aperture size The signal-to-noise ratio of the CT image is increased when a larger amount of x-ray photons are absorbed by the detectors. From the choices given, only an increase in aperture size (being width) what accomplish that.

Daily air or water calibration of the CT system is performed to evaluate which of the following components of image quality? a. contrast resolution b. temporal resolution c. uniformity d. linearity

d. linearity The accuracy between calculated CT numbers and their respective linear attenuation coefficients is termed linearity. Daily system calibration and routine vendor maintenance procedures work to establish and maintain CT system linearity.

Areas of a CT image that contain minimum changes in tissue density are electronically represented by: a. positive CT numbers b. hi spatial frequencies c. negative CT numbers d. low spatial frequencies

d. low spatial frequencies The contrast of the CT image is controlled by the spatial frequencies of the tissues within the section. Tissues have differing densities are represented electronically by different spatial frequencies. Adjacent tissues with similar densities or areas of tissue with minimal differences and density are represented by low spatial frequencies.

Which of the following archival media is capable of storing the largest number of CT images? a. floppy disc b. magnetic tape c. VHS tape d. optical disk

d. optical disk

The CT image is most accurate when a voxel represents only one tissue type. _____ occurs when multiple types of tissue are represented by a single voxel. a. edge gradiant b. binning artifact c. noise d. partial volume averaging

d. partial volume averaging The attenuation coefficients for each tissue type are averaged, yielding a single pixel value whose HU attempts to represent the entire contents of the voxel. Partial volume averaging can be reduced by minimizing the voxel dimension by reducing reconstructed section width. pg 91.

Which of the following terms may be used to describe the quantity of radiation emitted from the CT x-ray tube toward the patient? a. effective mAs b. photon flux c. constant mAs d. photon fluence

d. photon fluence Photon fluence maybe described as the quantity of x-radiation passing through a unit area. Constant mAs is the product of the selected mA setting and the scan time. Photon flux is the rate at which a quantity of x-radiation emitted from a CT tube passes through a unit area over a unit of time. Effective mAs is the calculated mAs applied to each reconstructed slice.

Which of the following formulas maybe used to calculate the dimensions of a pixel? a. pixel size = matrix size/DFOV b. pixel size = DFOV x matrix size c. pixel size = slice thickness/matrix size d. pixel size = DFOV/matrix size

d. pixel size = DFOV/matrix size

Which of the following statements regarding pre-detector collimation of the CT X-Ray beam is true? a. pre-detector collimation reduces patient radiation dose b. pre-detector collimation reduces the production of scatter radiation c. pre-detector collimation determines the scan field of view d. pre-detector collimation removes scatter radiation before it reaches the detectors

d. pre-detector collimation removes scatter radiation before it reaches the detectors The purpose of the pre-detector or postpatient collimation is to remove scatter radiation and to shape the portion of the beam that is incident on each detector.

The measurement of transmitted radiation made by an individual detector is called a(n): a. attenuation coefficient b. Hounsfield value c. CT number d. ray sum

d. ray sum The measurement of transmitted radiation made by an individual detector is called a ray sum. It equals the total attenuation occurring along a straight-line path from tube to detector.

Which of the following artifacts is not affected by the CT tech? a. motion b. partial volume c. edge gradient d. ring

d. ring Although the CT tech must be able to readily identify them, ring artifacts are caused by detector malfunction and are beyond his control. Patient motion, partial volume averaging, and edge gradient artifacts can all be limited by the CT tech through adequate prep and careful scan procedures.

Third-generation CT scanners use which of the following scan geometries? a. translate-rotate b. rotate-stationary c. transaxial d. rotate-rotate

d. rotate-rotate both the x-ray tube and detector array rotate around the patient during scanning with the third-generation CT unit.

The major disadvantage of the back projection method of image reconstruction is the appearance of the: a. partial volume effect b. beam hardening artifact c. Gibb phenomenon d. star artifact

d. star artifact The star artifact was an unwanted byproduct of the back projection method of image reconstruction used in older CT scanners. It is now removed by the process of convolution used in the modern reconstruction method known as filtered back-projection.

The streaking artifact that occurs in the area of the posterior fossa during the CT examination of the brain may be referred to as: a. cupping b. the "boiled egg" artifact c. stairstep d. the Hounsfield bar

d. the Hounsfield bar The dense bony areas of the posterior fossa of the skull are especially prone to beam hardening artifacts. Houndsfield bar is commonly used to describe this particular artifact.

In multi slice CT (MSCT), the combined thickness of all the sections simultaneously acquired with each gantry rotation are called the: a. detector configuration b. beam pitch c. detector pitch d. total collimation

d. total collimation Total collimation equals the combined thickness of all the sections that are simultaneously acquired with each gantry rotation. For example, a given MSCT system has an array of 64 detectors, each 0.625 mm wide. If the beam is collimated to expose the entire array, the total collimation for the acquisition is equal to 0.625 mm x 64, or 40 mm.

In the formula used to calculate the linear attenuation coefficient, the symbol "I" identifies: a. Euler constant b. incident intensity c. absorber thickness d. transmitted intensity

d. transmitted intensity The Lambert-Beer law is used to calculate the attenuation coefficient of the volume of material. The symbol "I" represents the intensity of the radiation passing through the tissue. It is compared with "Io" representing the intensity of the radiation incident upon the tissue being imaged during the calculation of the linear attenuation coefficient.

The 3D CT technique that includes all of the acquired voxel information in the reconstructed model with adjustments to its opacity is termed: a. surface rendering b. maximum intensity projection (MIP) c. curved multiplanar reformation d. volume rendering

d. volume rendering Volume rendering adjusts the opacity of voxels included in the 3D model according to their tissue characteristics. Unlike the thresholding concept used for shaded-surface display (SSD), or surface rendering, volume rendering does not exclude voxels, but instead alters their appearance so that the 3D model contains the entire volume dataset.

The process of grayscale mapping of the CT image may be referred to as: a. analog to digital conversion b. retrospective reconstruction c. prospective reconstruction d. windowing

d. windowing Windowing is used to describe the process of grayscale mapping the CT image, during which the display system assigns a shade of gray to an individual pixel on the basis of its CT number (HU).

The mathematical process that allows MDCT images to be reconstructed at any point along the acquired volume is commonly referred to as: a. iteration b. filtered back-projection c. Fourier reconstruction d. z-filtering

d. z-filtering Multiple-detector row CT systems utilize the process called z-filtering during image reconstruction. The filtering allows for thin sections to be reconstructed at any point along the acquired Z-axis volume. This interpolation technique utilizes multiple complementary rays beyond those immediately above and below the particular slice plane.