Mosby CT Review Exam: Image Production

Which of the following statements best describes why the image of the chest is displayed at a window whose level is -700 and width is 1500? See Fig. A. CT images of the lung should always be displayed in predetermined "lung" windows. B. Soft tissue demonstration is not required during the CT evaluation of the chest. C. Strict protocols regarding image display should never be altered by the operator. D. The level is set at the average Hounsfield value for the tissue of interest within a range determined by the window width.

.D. The level is set at the average Hounsfield value for the tissue of interest within a range determined by the window width. The window chosen for image display should always be tailored to meet the needs of the individual anatomy. The level should be set at an average value for the tissue of interest with a width wide enough to include all variations within the area of interest.

Which of the following azimuth settings was used to produce the localizer image? See Fig. A. 0 degrees B. 90 degrees C. 180 degrees D. 270 degrees

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

Two adjacent pixels are measured to have a difference of 1 HU. 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%.

The image was produced with a 1.0-mm aperture size and was displayed using a 5122 matrix and a 15-cm DFOV. The voxel dimension for this image would be: See Fig. A. 0.29 mm × 0.29 mm × 1.0 mm. B. 2.9 mm × 2.9 mm × 1.0 mm. C. 3.4 mm × 3.4 mm × 1.0 mm. D. 0.29 mm × 0.29 mm × 1.0 cm.

A. 0.29 mm × 0.29 mm × 1.0 mm. The pixel dimension 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. (15cmx10 = 150mm, then 150mm/512 = 0.29mm)

Which of the following technical adjustments may be employed to improve the temporal resolution of an MDCT system? 1. Decreased scan time 2. Decreased section width 3. Decreased DFOV A. 1 only B. 1 and 2 only C. 2 and 3 only D. 1, 2, and 3

A. 1 only Temporal resolution quantifies the CT system's ability to freeze motion and to provide an image free of blurring. The controlling factors of the temporal resolution of a CT system are the gantry rotation speed and reconstruction method.

Which of the following is not a typical matrix size used with a modern CT scanner? A. 80✕80 B. 320✕320 C. 512✕512 D. 1024✕1024

A. 80✕80 Modern CT scanners use large matrices for image reconstruction and display. Typical sizes include 3202, 5122, and 10242.

Which of the following terms best describes the artifact present in the image? See Fig. A. Edge gradient B. Gibbs phenomenon C. Density gradient D. Partial volume

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

Which of the following types of algorithms would be best suited to demonstrate the bony details of the shoulder joint in the figure? See Fig. A. High spatial frequency B. Soft tissue C. Standard D. Low spatial frequency

A. High spatial frequency High spatial frequency algorithms demonstrate the greatest spatial resolution and are best suited for imaging sharp density changes, such as those occurring with bone tissue. Bone, edge, and detail are names commonly given to high spatial frequency algorithms.

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.

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. Cormack

A. Radon J. Radon, an Austrian mathematician, was responsible for some of the earliest scientific research for the reconstruction principles used in computed tomography. 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 technical adjustments would result in a decrease in the signal-to-noise ratio (SNR)? A. Reducing the section width B. Decreasing the detector pitch C. Selecting a smooth algorithm over a bone algorithm D. Increasing in the display field of view (DFOV)

A. Reducing the section width Any decrease in the voxel dimension increases noise or decreases the signal-to-noise ratio (SNR).

Which of the following types of gas 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 minimum object size that a CT scanner can resolve may be calculated by taking the reciprocal value of the scanner's 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 may be divided in half to provide the minimum object that this scanner can resolve: 0.3 mm.

A CT image is reconstructed using a 5122 matrix and a 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 may be calculated by dividing the field of view by the matrix size. The DFOV of 17 cm must first be converted into 170 mm. This is then divided by 512 mm, for a pixel dimension of 0.33 mm. Keep in mind that the pixel is a two-dimensional item, square in shape, and the measurement of 0.33 mm corresponds to only one side.

Which of the following components of CT image quality is/are being evaluated? See Fig . 1. Linearity 2. Section width 3. Spatial resolution A. 1 only B. 1 and 2 only C. 2 and 3 only D. 1, 2, and 3

B. 1 and 2 only A linearity phantom contains four inserts of various materials surrounded by water. Known ranges of CT values are compared with those obtained by CT acquisition to ensure system accuracy and linearity. This American College of Radiology (ACR) phantom also contains two ramps consisting of a series of wires spaced 0.5 mm apart along the phantom's z-axis. The accuracy of the selected section width is evaluated by counting the number of wires visible in cross section.

Soft copies of CT images are stored on which of the following media? 1. Hard drives 2. Magnetic optical disk 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 in digital file format is referred to as soft copies. Hard copies of CT images are those stored on laser film.

Which of the following may be used for the archival storage of CT images? 1. CD-ROM 2. VHS tape 3. Magnetic optical disk A. 1 only B. 1 and 3 only C. 2 and 3 only D. 1, 2, and 3

B. 1 and 3 only CD-ROM and magnetic optical disk (MOD) are two media commonly employed for CT image archival.

Each 1.0% in contrast between adjacent objects amounts to a difference in pixel value of approximately: A. 3 HU. B. 10 HU. C. 25 HU. D. 60 HU.

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

Which of the following steps could be taken to reduce the artifact? See Fig. 1. Reduce section thickness. 2. Angle gantry around fillings. 3. Decrease kVp. A. 1 only B. 2 only C. 3 only D. 2 and 3 only

B. 2 only The only effective method of reducing streak artifact because of metal is to remove the metal object(s) from the scanning field. In this case, the dental fillings cannot be removed, but the gantry could be angled to avoid their interference.

What is the display field of view used for a 320^2 (squared; to the 2nd power) matrix image with a pixel dimension of 0.75 mm ✕ 0.75 mm? A. 12 cm B. 24 cm C. 36 cm D. 48 cm

B. 24 cm The display field of view may be 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 3202. The DFOV is 240 mm or 24 cm.

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-ray photons that reach the patient.

The patient scanned in the image measured 38 cm across. An appropriate DFOV for the display of this image would be: See Fig. A. 34 cm. B. 40 cm. C. 44 cm. D. 48 cm.

B. 40 cm. The display field of view chosen for an image should be large enough to include the entire area of interest. A DFOV that is significantly larger than the anatomic area of interest will result in a minified image with reduced spatial resolution.

The human eye is capable of differentiating approximately __________ shades of gray. A. 10 to 25 B. 60 to 80 C. 100 to 120 D. 250 to 256

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

Which of the following would increase the noise apparent on a CT image? A. Increase in mAs B. Decrease in aperture size C. Increase in filtration D. Decrease in matrix size

B. Decrease in aperture size A decrease in 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 collimators used to adjust the size of the aperture absorb radiation as they restrict the beam.

CT acquisition of a water-filled phantom is performed utilizing a standardized set of technical factors. If a region-of-interest (ROI) measurement is made, the image noise corresponds to which of the following data? A. Mean CT number B. Standard deviation C. CT number mode D. Median Hounsfield value

B. Standard deviation Noise is most commonly measured by scanning a water-filled phantom with a standardized set of technical factors. The image noise is equal to the standard deviation of pixel values within a region-of-interest (ROI) measurement of the image.

The ability of the CT system to reduce involuntary motion artifacts and provide CT images free of motion artifact is determined by which of the following components of image quality? A. In-plane spatial resolution B. Temporal resolution C. Longitudinal spatial resolution D. Contrast resolution

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

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 reconstruction may be used to adjust the center of the image.

B. The slice thickness and SFOV may be changed. Retrospective reconstruction 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 multidetector CT (MDCT) technology has made retrospective adjustments in section width possible. The scan field of view (SFOV) remains an item that may not be altered retrospectively.

Errors during the measurement of transmitted radiation by the detectors can result in a form of noise on the image referred to as a(n): A. fluence. B. artifact. C. line spread. D. encryption.

B. artifact. An artifact is a form of noise that occurs from an error during data acquisition or CT image display.

The streaking artifacts present were most likely caused by: See Fig. A. tube arcing. B. dental fillings. C. detector malfunction. D. insufficient technique.

B. dental fillings. Metallic items such as dental fillings are a common cause of streaking or "star" artifact.

A straight line appearing vertically on the scanogram (pilot) of a fourth-generation CT scanner is an artifact most likely caused by: A. edge gradient. B. detector malfunction. C. tube arcing. D. beam hardening

B. detector malfunction. A straight line artifact on a scanogram (pilot) image is most likely caused by a malfunctioning detector in both third- and fourth-generation scanners. As the patient travels through the gantry, the faulty detector does not record any information, leading to the blank line that appears on the image.

The technique that provides the detail of the outer portions of the 3-D model while maintaining the typical CT detail of the lungs inside is called: See Fig. 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 surface-rendered 3-D model provides excellent surface anatomy while maintaining the normal cross-sectional anatomy appearance of the volume inside the model.

The artifact present was most likely caused by: See Fig. A. patient motion. B. surgical staples. C. gallstones. D. detector malfunction.

B. surgical staples. The streaking artifact in the right anterior portion of the image was most likely caused by surgical staples. This type of metallic streaking artifact is caused by the edge gradient effect.

Which of the following technical factors exhibits an effect upon longitudinal spatial resolution? 1. Detector pitch 2. Interpolation algorithm 3. Display field of view (DFOV) 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 algorithm inherent to the system and the selected detector pitch.

Which of the following corresponds to the full width at half maximum of the graph? See Fig. A. A B. B C. C D. D

C. C The full width at half maximum (FWHM) of the SSP curve demonstrates the effective section width of a volumetric CT acquisition. It is measured by examining the slice sensitivity profile at half of its maximum height.

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 at 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 a result. The interface between dense bone and soft tissue is a common site for the edge gradient effect to occur.

In 1979, the scientists __________ and __________ shared the Nobel Prize for their research in computed tomography. A. Watson and Crick B. Olendorf and Hounsfield C. Hounsfield and Cormack D. Hounsfield and Ambrose

C. Hounsfield and Cormack Dr. Godfrey Hounsfield and Dr. Allan Cormack shared the Nobel Prize for their work on the development of computed tomography in 1979.

The term beam hardening is used to describe which of the following physical phenomena? A. The decrease in average photon energy of a heterogeneous x-ray beam B. The increase in average photon energy of a homogeneous x-ray beam C. The increase in average photon energy of a heterogeneous x-ray beam D. The decrease in average photon energy of a homogeneous x-ray beam

C. The increase in average photon energy of a heterogeneous x-ray beam Beam hardening occurs as the 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 and the lower-energy photons are absorbed.

The streaking artifact present is most likely because of: See Fig. A. detector malfunction. B. tube arcing. C. beam hardening. D. out-of-field errors.

C. beam hardening. Streaking artifacts such as the one in the figure commonly occur during CT scanning of thick body parts like the shoulder. The x-ray beam changes in quality as it passes through different densities. These beam energy changes sometimes manifest as streak artifacts.

The artifact present is most likely caused by: See Fig. A. involuntary motion. B. aliasing. C. surgical staples. D. tube arcing

C. surgical staples. High-density objects such as surgical clips and other types of prosthetic devices are often the cause of streaking artifacts because of the edge gradient effect.

The high-density objects labeled as Number 2 most likely represent: See Fig. A. gunshot fragments. B. loose change in the patient's clothing. C. surgical staples. D. ingested metallic material.

C. surgical staples. The dense objects located in the abdomen of this patient most likely represent metallic surgical staples, which often appear in the post-surgical patient.

The ability of a CT system to maintain consistent Hounsfield 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.

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 multislice CT (MSCT) system utilizes a third-generation curvilinear detector array with multiple rows of individual detector elements segmented along the longitudinal axis, or z-axis.

The Hounsfield value of a pixel is directly related to which of the following? A. Window width B. Field of view size C. μ of H2O D. Window level

C. μ of H2O The value of a pixel in Hounsfield units is calculated through a comparison of the linear attenuation coefficient (μ) of a voxel of tissue to that of water.

Which of the following window levels was most likely used to display the image shown? See Fig. A. -700 HU B. 0 HU C. +50 HU D. +250 HU

D. +250 HU The window level used to display an image should equal the average Hounsfield value of the tissue of interest. In this example, the demonstration of bone tissue is most important. The type of bone making up the shoulder joint would have an average density of +250 HU.

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. 2 and 3 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 or completely eliminates broadening of the SSP and also improves longitudinal spatial resolution.

Important characteristics of a quality control program in CT include: 1. Consistent testing frequency 2. Prompt corrective actions when necessary 3. Accurate record-keeping A. 1 only. B. 1 and 2 only. C. 1 and 3 only. D. 1, 2, and 3

D. 1, 2, and 3 The hallmarks of a superior QC program are routine frequency of testing, prompt corrective actions as needed, and accurate record-keeping.

Which of the following section widths would cause the greatest amount of partial volume averaging? A. 1 mm B. 3 mm C. 5 mm D. 10 mm

D. 10 mm Large section widths result in long voxel lengths with possibly several tissue types accounted for within the voxel. When a CT number is assigned to the representative pixel, the density of all of the tissue types is averaged, yielding one attenuation coefficient. Narrow section widths tend to decrease the partial volume effect.

Which of the following term(s) is/are 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 densities is termed contrast resolution or sensitivity. A CT scanner with poor 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/are correct? 1. 360-degree 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 detector(s) translated across the patient's head, recording transmission measurements. The entire system then rotated 1 degree. This process of translation-rotation then repeated itself for a total of 180 degrees. First- and second-generation CT scanners used anywhere from 2 to 30 detectors.

A 512✕512 matrix will consist of how many pixels? A. 512 B. 1024 C. 26,214 D. 262,144

D. 262,144 The matrix size indicates the number of pixels across and the number of pixels down. A 512✕512 matrix has 262,144 pixels, which is calculated by taking the square of 512.

In a modern CT system, the total number of possible Hounsfield values that may be assigned to any one pixel is approximately: A. 512. B. 1024. C. 2000. 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 to the 12th power = 4096).

Which of the following will serve to decrease the noise of a CT image? A. Decrease 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 dimension. This larger size causes an increase in the number of x-rays passing through each pixel, thereby increasing the signal-to-noise ratio.

Which of the following components of the CT system is the most common cause of ring artifacts on the reconstructed CT image? A. x-ray tube B. Collimator C. Slip-ring D. Detector

D. Detector Ring artifacts are typically caused by faulty detectors. An error in a detector results in the back-projection of an incorrect ring of density on the reconstructed CT image.

Which of the following technical changes may increase the partial volume effect present on a spiral CT scan? A. Decrease in pitch. B. Decrease 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 or the table speed has been increased. The effects of increased section width on the partial volume effect are obvious. The increased table speed also plays a role in increasing partial volume averaging. Because the table moves 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 section sensitivity profile (SSP), which manifests as partial volume averaging.

Which of the following archival media is capable of storing the largest number of CT images? A. Floppy disk B. Magnetic tape C. VHS tape D. Optical disk

D. Optical disk Magnetic optical disks vary in storage capacity, with some capable of storing several thousand 5122 matrix CT images. This is considerably more than magnetic tape, which had previously been the standard choice for CT image archival.

Which of the following anatomic quadrants has been removed from the 3-D model? See Fig. A. Left posterior inferior B. Right anterior inferior C. Left anterior superior D. Right anterior superior

D. Right anterior superior The computer software is able to remove quadrants of information to allow visualization of the inner portions of the model. In this example, the right anterior superior portion has been removed. This information can be ascertained by examining the anatomic position of the model. In the 3-D model, the information is also available on the image from the letters RAS, which signify "right anterior superior."

The graph represents which of the following? See Fig. A. Modulation transfer function (MTF) B. Signal-to-noise ratio (SNR) C. Views per rotation (VPR) D. Slice sensitivity profile (SSP)

D. Slice sensitivity profile (SSP) The section width (slice) for a volumetric acquisition may be graphically displayed as a slice sensitivity profile (SSP). The SSP represents the degree of broadening that occurs along the z-axis during volumetric data acquisition.

The ROI measures -987 HU. This phantom insert most likely represents: See Fig. A. bone. B. acrylic. C. polyethylene. D. air

D. air Linearity may be periodically evaluated by scanning a specialized phantom containing samples of an assortment of materials—acrylic, polyethylene, water, air, artificial bone, and so on. A CT number measurement of -987 HU would indicate that the particular phantom insert represents air.

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 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 and may result in a loss of system uniformity.

Cupping artifacts 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 in density 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 site for cupping artifacts to occur.

The full width at half maximum (FWHM) of a slice sensitivity profile diagram indicates the: A. limiting resolution. B. signal-to-noise ratio (SNR). C. dose profile. D. effective section width.

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

The spatial resolution of a CT scanner is often measured using the MTF of the system, which is an acronym for: A. maximum transmissivity frequency. B. modulation target function. C. minimum transmissivity frequency. D. modulation transfer function.

D. modulation transfer function. The modulation transfer function (MTF) is a mathematical method of quantifying the spatial resolution of a CT scanner.

A CT image of a homogeneous material contains variations in CT number from pixel to pixel. This image is said to have: A. high contrast. B. sensitivity. C. definition. D. noise.

D. noise. A CT image of a homogeneous material should have pixels with the same CT number. Any variation in CT number between pixels indicates that noise has entered the system, causing a loss in accuracy.

The degree of CT section broadening that occurs along the z-axis during volumetric data acquisition is represented graphically by the: A. modulation transfer function (MTF). B. Nyquist theorem. C. signal-to-noise ratio (SNR). D. slice sensitivity profile (SSP).

D. slice sensitivity profile (SSP). The SSP represents the degree of broadening, or point spread, that occurs along the z-axis during volumetric data acquisition.

Which of the following mathematical techniques is used for the reconstruction of volumetric MDCT images? A. 180-degree interpolation (180LI) B. 360-degree interpolation (360LI) C. Back-projection D. Fourier reconstruction

A. 180-degree interpolation (180LI) The 180-degree linear interpolation technique (180LI) 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 acquisitions may be characterized as overlapping? 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

A. 2.5-mm sections reconstructed every 1.25 mm Overlapping images are reconstructed with a section interval that is less than the section width. A percentage value may be assigned to the degree of overlap. For example, 2.5-mm-thick images reconstructed every 1.25 mm are said to possess a 50% overlap.

Which of the following manipulations involves the use of image data? A. Adjusting the width and level of a window setting B. Decreasing the DFOV C. Changing the algorithm selection D. Increasing the matrix size

A. Adjusting the width and level of a 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. Adjustments in the algorithm, DFOV, or matrix require the raw data so that the image may be retrospectively reconstructed.

The interaction between x-ray and matter that is responsible for the production of the scatter radiation absorbed by the patient and detectors is: A. Compton effect. B. bremsstrahlung. C. photoelectric. D. characteristic

A. Compton effect. In the Compton interaction, an x-ray photon ejects an outer-shell electron of an atom. The photon loses some of its energy in the collision and then continues on in a different, scattered direction. This interaction is the major source of the scatter radiation involved in the formation of the CT image.

A voxel's dimension may be decreased by which of the following? A. Decreasing the section width B. Decreasing the matrix size C. Increasing the section width D. Increasing the DFOV

A. Decreasing the section width The section width of a CT image controls the length of the voxel. The dimensions of a voxel may also be reduced through decreases in the size of the pixel. Increases in matrix size and decreases in DFOV serve to decrease the dimensions of the pixel and voxel.

The graph is used to evaluate which of the following components of CT image quality? See Fig. A. Spatial resolution B. Signal-to-noise ratio (SNR) 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 because of adjustments in technical factors.

The phantom imaged is used to test which of the following image quality factors of a CT scanner? See Fig. A. Spatial resolution B. Noise C. Linearity D. Contrast resolution

A. Spatial resolution The resolution test pattern embedded within this type of phantom is used to measure the spatial resolution of the CT scanner. The effects of different technical factors on spatial resolution may be examined by calculating the maximum number of visible line pairs.

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. Detector configuration refers to the number, length, and organization of the individual detector elements in an MSCT system.

For a given CT acquisition, the calculated mAs applied to each reconstructed slice may be 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 slice. The primary influencing factor for effective mAs is table speed, which is an important component controlled by the selected detector pitch. The 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.

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 termed the linear attenuation coefficient (μ).

The portion of the primary beam interacting with a single detector is known as a: A. ray. B. view. C. profile. D. sample.

A. ray. The term ray is used to describe the portion of the x-ray beam that falls upon a single detector.

The technique that allows the user to select the range of pixel values used in a 3-D CT reformation is termed: A. thresholding. B. windowing. C. retrospective reconstruction. D. targeting

A. thresholding. The selected threshold allows the user to select the range of pixel values rendered in the 3-D model. For example, a higher, bone threshold (>300 HU) can be chosen to build a skeletal model of the patient's skull. Decreasing the threshold to include voxels with lower attenuation values results in a 3-D model with the patient's soft tissues included.

The CT number is a relative value that standardizes the attenuation value (μ) of a tissue by comparing it with the attenuation of: A. water. B. air. C. bone. D. a standard Plexiglass phantom.

A. water. The CT number is a relative value that standardizes the attenuation value (μ) of a tissue by comparing it with the attenuation of water for a particular CT system.

Which of the following quality control tests should be performed daily on a 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 tests for CT number accuracy and noise levels should be performed daily on CT scanners. The laser localization system of a CT scanner need be checked only once per year.

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 (MTF) 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 falls somewhere in between.

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 ✕ 0.625 = 20 mm).

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

Which of the following acquisitions may be characterized as noncontiguous? 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

B. 5.0-mm sections reconstructed every 7.5 mm Noncontiguous images are images acquired with an interval greater than the section width. For example, 5.0-mm sections reconstructed every 7.5 mm fail to evaluate the intermittent 2.5 mm of tissue.

Which of the following terms accurately describes the type of x-ray beam used in a third-generation CT scanner? A. Pencil beam B. Fan beam C. Nutating beam D. Electron beam

B. Fan beam The third-generation CT scanner uses a fan beam projected onto a wide detector array. The third-generation CT x-ray beam is commonly mistaken for the "pencil beam" used with only two detectors in the first-generation CT scanner.

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 of CT image reconstruction include simultaneous reconstruction, ray-by-ray correction, and point-by-point correction. The Fourier transform method is an analytic method of CT image reconstruction.

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 (DFOV).

B. Increase in sampling frequency. The sampling frequency, or views per rotation (VPM), controls the volume of transmission data acquired for each gantry rotation. Very small objects exhibit high spatial frequencies that only systems with high sampling rates will be capable of resolving.

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 first-generation prototype CT scanner designed by Dr. Godfrey Hounsfield used an iterative form of image reconstruction.

The dimensions of a voxel may be calculated as the product of which of the following? A. Matrix size and pixel size B. Pixel size and section width C. DFOV and matrix size D. DFOV and pixel size

B. Pixel size and section width The pixel is a two-dimensional representation of a voxel. The section width is equal to the length of the voxel. To calculate the dimensions of a voxel, the pixel dimension must be multiplied by the section width.

Which of the following is used in gas ionization CT detectors? A. Neon B. Xenon C. Helium D. Nitrogen

B. Xenon Gases with high atomic numbers, such as xenon, are used in ionization-type CT detector systems. The high atomic number of the xenon gas (Z = 54) increases the incidence of interaction with x-ray photons, thus improving the efficiency of the detector.

The following formula is used to calculate the linear attenuation coefficient: I = I0e-μx The symbol "x" 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 a volume of material. The symbol "x" represents the thickness of the absorber attenuating the radiation.

The computer technique used to reduce the size and storage requirements of digital CT image data is called: A. archival. B. compression. C. encryption. D. rendering

B. compression. Image compression is a complex computer technique that reduces the size of digital CT image data. Compression of CT images reduces the storage requirements and increases the speed at which CT images may be transmitted across computer systems.

The term __________ describes the ability of a CT scanner to differentiate objects with minimal differences in attenuation coefficients. A. spatial resolution B. contrast resolution C. linearity D. modulation

B. contrast resolution The contrast resolution of a CT scanner refers to the system's ability to differentiate objects with similar densities. This ability is also commonly referred to as the low-contrast resolution or the sensitivity of the CT scanner.

A test phantom containing water is scanned and five region-of-interest measurements are performed. The subsequent density measurements are compared and demonstrate a maximum deviation of less than 2 HU. This quality assurance test was performed to evaluate the scanner's: A. spatial resolution. 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 a CT scanner. The ROI measurements are arranged with one at the center of the image and the others rotated about the periphery. Each of the five ROI measurements taken from the water phantom should yield relatively the same CT number value.

Areas of a CT image containing abrupt changes in tissue density are electronically represented by: 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 tissue(s) within the section. Tissues of differing densities are represented electronically by different spatial frequencies. Adjacent tissues with large differences in density are represented by a high spatial frequency signal.

Which of the following factors has no measurable effect on spatial resolution? A. Focal spot size B. kVp C. Detector sampling frequency D. Matrix size

B. kVp The spatial resolution of a CT image depends on several factors, including focal spot size, detector size and spacing, field of view, pixel dimension, and detector sampling rate.

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 can negatively affect image detail.

The spatial resolution of a CT scanner is usually given in the unit of measure: A. HU. B. lp/cm. C. µm. D. Hz.

B. lp/cm. The spatial resolution of a scanner quantifies its ability to visually separate small objects in the image. The unit used to describe the spatial resolution of a CT scanner is the lp/cm. Spatial resolution is tested by scanning a phantom with an embedded resolution test pattern consisting of a series of small lead lines that decrease in size and spacing. The smallest, most closely spaced set of lead lines is said to be the spatial resolution of the CT scanner. A line pair (lp) consists of one lead line and the space immediately next to it.

The grainy appearance of the pelvic image shown is commonly referred to as: See Fig. A. tube arcing. B. noise. C. edge gradient. D. partial volume artifact.

B. noise. Noise degrades the CT image, giving it a grainy, mottled appearance. The terms quantum mottle and noise are often used interchangeably.

The largest deterrent of a CT scanner's 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.

The component of the DAS responsible for strengthening the signal emitted from a detector is termed the: A. digital-to-analog converter (DAC). B. preamplifier. C. high-resolution comb. D. bow-tie filter.

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 preamplifier is the component of the data acquisition system (DAS) responsible for amplifying the detector 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 may be calculated by multiplying the pixel dimension (mm2) by the: A. display field of view (DFOV). B. section width. C. pitch. D. scan field of view (SFOV).

B. section width. As a geometric cuboid, the volume of a voxel may be calculated as the product of the pixel area dimension (mm2) and the section width (mm). The pixel dimension (d) must be first calculated by dividing the display field of view (DFOV) by the matrix size. The area of the pixel may then be found by squaring the value of d (mm). The unit of measurement for voxel volume is mm3.

The ability of a CT scanner 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 assignment of different generations to CT scanners is based on the configuration of the: A. patient and gantry. B. tube and detectors. C. anode and cathode. D. tube and collimators

B. tube and detectors. CT scanners are often placed into one of several different "generations." The differences in generations are based on the relationship of the tube and detectors and the position of each during data acquisition.

When one is using a third-generation CT scanner, it is important that reference detectors positioned at the peripheral portions of the detector array be exposed to: A. homogeneous radiation. B. unattenuated radiation. C. monochromatic radiation. D. remnant radiation.

B. unattenuated radiation. Reference detectors are used to measure incident radiation intensity. This information is used by the computer during the calculation of the linear attenuation coefficient. If the patient is placed incorrectly within the gantry, the reference detectors may be partially blocked, causing an out-of-field artifact.

During a CT scan, each sample of ray sum measurements made by the data acquisition system (DAS) 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. During the process of data acquisition, views are acquired by the DAS hundreds of times per second, according to the system's individual sampling rate.

A CT system measures the average linear attenuation coefficient of a voxel of tissue to be 0.189. The linear attenuation coefficient of water for this scanner equals 0.181. The CT number assigned to the pixel representing this voxel of tissue equals: A. -956 HU. B. -173 HU. C. +44 HU. D. +1044 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 a contrast factor of 1000 to yield the value of the pixel in Hounsfield units.

The average CT value for blood is approximately: A. -50 HU. B. 0 HU. C. +45 HU. D. +100 HU

C. +45 HU. Blood has an approximate CT number value of +45 Hounsfield units.

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 +3071 HU. D. -4096 HU and +4096 HU.

C. -1024 HU and +3071 HU. The typical range of Hounsfield scale CT numbers is -1024 to +3071 HU. This range equals the 4096 different values possible when one is using a 12-bit system (212 = 4096).

Which of the following technologic advances has led 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 Spiral/helical CT scanners are a recent development in computed tomography technology. The advents of slip-ring technology and improvements in x-ray tube design have made this innovation possible. Slip-rings have taken the place of the cumbersome cables previously used to transmit the CT signal and supply power to the tube and detectors. This development enables the tube and detectors to rotate continuously around the patient, acquiring data in the form of a helix. The longer exposure times of up to 60 seconds require extremely efficient x-ray tubes with enormous heat capacities.

Which of the following technical adjustments would decrease the quantum noise of a 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 a 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 (decreased 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 computed tomography are scintillation and gas ionization detectors. Both operate by measuring the amount of transmitted radiation passed through the patient and transmitting this information 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 x 1.25-mm detector elements along the z-axis. With a selected beam width of 20 mm, what beam pitch would result in the table moving 35.00 mm for each rotation of the gantry? A. 1.0 B. 1.5 C. 1.75 D. 2.0

C. 1.75 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 16 multiplied by 1.25 mm. The beam pitch may be therefore calculated by dividing 35.00 by 20, or 1.75.

An MDCT image is reconstructed using a 5122 matrix and a 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. 0.21 mm3 B. 1.85 mm3 C. 2.15 mm3 D. 3.36 mm3

C. 2.15 mm3 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 mm2. The volume of the voxel may be calculated by multiplying the pixel area by the section width.

What is the maximum number of line pairs per centimeter (lp/cm) the smooth algorithm can demonstrate? See Fig. A. 1.0 B. 2.0 C. 8.1 D. 9.0

C. 8.1 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 MTF is lower than 10% (0.1), the object is no longer resolved.

A voxel may be defined as which of the following? A. The portion of the CRT displaying the image B. A miniature image C. A volume element D. An arrangement of pixels

C. A volume element A voxel may be defined as a volume element. It is represented within a matrix by a pixel.

Which of the following is the primary interaction between x-ray photons and tissue during computed tomography? A. Bremsstrahlung B. Characteristic C. Compton effect D. Coherent scatter

C. Compton effect Owing to the high-energy beam used in CT, the Compton effect is the predominant interaction between x-ray and matter. CT scanners use between 120 and 140 kVp, yielding x-ray energies with averages between 70 and 80 keV.

During CT angiography, images may be reconstructed using only the greatest density encountered along each ray. This type of specialized CT image is called a(n): A. 3-D model. B. volume-rendered image. C. MIP image. D. surface-rendered image.

C. MIP image. Maximum intensity projections (MIPs) are commonly used during specialized studies such as CT angiography. An MIP image is constructed by displaying only the maximum intensity pixel found along each ray. Remember that a ray is the path from the focal spot of the x-ray tube to a detector. This technique is valuable in displaying contrast-enhanced blood vessels that are surrounded by various types of tissue.

Which of the following types of 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) and with increases in the pixel and voxel dimensions.

Which of the following is not used to archive CT images? A. 3.5-inch floppy disk B. Magnetic tape C. VHS tape D. Magnetic optical disk

C. VHS tape VHS tape is common video tape used to record dynamic images such as motion pictures from a video camera. CT images are digital static images that may be archived on floppy disk, magnetic tape, CD-ROM, and magnetic optical disk.

A pixel may be defined as: A. the portion of the CRT displaying the image. B. a volume element. C. a picture element. D. a miniature image

C. a picture element. A pixel is an element of the digital image. It is located in a matrix and is a two-dimensional representation of a voxel. When CT numbers are calculated for voxels (volume elements) of tissue, the pixel is assigned the calculated number and a subsequent shade of gray. The pixel may be defined as a picture element, or the smallest component of the digital image.

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 about the patient, the signal varies according to the measured x-ray energy flux.

The type of compensating filter used at the x-ray tube of a CT scanner is called a: A. kernel. B. water bath. C. bow-tie filter. D. wedge filter

C. bow-tie filter. The bow-tie filter used at the x-ray tube of a CT scanner absorbs a larger amount of radiation at the periphery of the patient, where the part thickness is reduced. The center of the patient is placed at the center of the filter, where the largest amount of radiation is allowed to pass through. The use of this type of filter attempts to compensate for the differences in thickness of the often oval-shaped patient.

The type of MSCT detector array that contains midline narrow elements flanked by wider detectors is called a(n): A. uniform matrix array. B. adaptive array. C. hybrid array. D. stationary array.

C. hybrid array. A hybrid array consists of two detector sizes. The narrower detectors are positioned midline, flanked by the wider detectors. For example, the central rows of a 16-channel MDCT system are 0.625 mm with the remaining peripheral rows at 1.25 mm.

A voxel whose attenuation coefficient is less than that of water is assigned a pixel value with a(n) __________ CT number. A. positive B. extremely large C. negative D. invalid

C. negative The attenuation coefficient of a tissue describes the tissue's ability to attenuate x-radiation. CT numbers are assigned to pixels based on the basis of the attenuation of the tissue within the voxel. The assignment of a CT number in Hounsfield units arises from a comparison of the attenuation coefficient of the tissue with that of water. Materials with attenuation coefficients less than that of water are assigned negative CT numbers.

The computerized communication system used by a PACS to transmit data is referred to as a: A. convolution. B. hierarchy. C. network. D. matrix

C. network. A PACS communicates with the CT system(s) and other modalities and peripherals via a computerized communication system, or network.

CT images that have been reconstructed from a portion of the data acquisition process in the 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-degree rotation of the x-ray tube. A CT image can also be constructed from a portion of the data acquisition phase. For example, if a patient moves during the last third of a 2-second scan, an image can be constructed from the first 240 degrees of tube rotation, yielding an image free of motion. This process, referred to as segmentation, is a software capability of many modern CT scanners.

The Digital Imaging and Communications in Medicine (DICOM) protocol was developed to: A. protect patient confidentiality. B. reduce patient radiation dose from CT imaging. C. standardize the networking and storage of digital images. D. increase the availability of cardiac CT.

C. standardize the networking and storage of digital images. The Digital Imaging and Communications in Medicine (DICOM) standard conforms the process of recording, storing, printing, and transmitting medical image data. The purpose of DICOM conformance is to ensure that all medical image files may be conveniently archived and transmitted among various systems and networks, regardless of the manufacturing vendor.

The process by which electrons are produced at the cathode of a CT x-ray tube is known as: A. rectification. B. anode heel effect. C. thermionic emission. D. isotropic emission.

C. thermionic emission. When the cathode of an x-ray tube is heated, electrons are released through the process of thermionic emission. It is these electrons that are then accelerated toward the anode for x-ray production.

In multislice CT (MSCT), 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 multislice 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 of the sections that are simultaneously acquired with each gantry rotation.

Which of the following corresponds to the longitudinal dimension of the CT image voxel? A. x-axis B. y-axis C. z-axis D. Orthogonal axis

C. z-axis The longitudinal dimension of the voxel corresponds to the z-axis of scanning, with a voxel depth equal to the reconstructed section width.

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

D. +1000 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 a contrast factor of 1000 to yield the value of the pixel in Hounsfield units.

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 only D. 1, 2, and 3

D. 1, 2, and 3 Each component of 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 may be altered by slice thickness, algorithm selection, and noise. The noise level of an image is dependent upon several factors, including patient dose and pixel dimension. Each of these factors must be considered by the CT technologist in an effort to provide high-quality CT studies.

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. 2 only B. 1 and 2 only C. 2 and 3 only D. 1, 2, and 3

D. 1, 2, and 3 The line spread function (LSF), point spread function (PSF), and modulation transfer function (MTF) are each used to quantify the spatial resolution of a CT scanner. The LSF measures the ability of a CT scanner to clearly image an edge or 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 varying densities. The MTF is derived from the measurement of the LSF and PSF.

Which of the following matrices would provide the greatest spatial resolution? A. 256 × 256 B. 320 × 320 C. 512 × 512 D. 1024 × 1024

D. 1024 × 1024 A larger matrix size consists of an increased number of pixels. The computer is then able to assign less information into each pixel, allowing the image to display extremely small objects separately from one another. This increases the spatial resolution of the CT scanner.

Which of the following reduces the scatter radiation reaching the detectors? A. Post-patient collimation B. Pre-patient collimation C. Pre-detector collimation D. Both a and c

D. Both a and c Pre-detector collimation and post-patient collimation are synonymous. Both terms describe a device designed to remove scatter radiation before it reaches the detector.

What is the dimension of each pixel in the matrix of an image with the following parameters? FOV = 25.6 cm Matrix = 512 mm ✕ 512 mm A. 0.5 mm B. 0.5 cm C. 0.05 cm D. Both a and c

D. Both a and c The dimension of a pixel may be calculated by dividing the field of view (FOV) by the matrix size. The DFOV, 25.6 cm, must first be converted into 256 mm. This is then divided by 512 mm for a pixel dimension of 0.5 mm. Keep in mind that the pixel is a two-dimensional item, square in shape, and the measurement of 0.5 mm corresponds to only one side. The numerical lengths of 0.5 mm and 0.05 cm are equal.

Which of the following technical factor adjustments will result in a reduction in patient radiation dose? A. Increase milliamperage (mA). B. Decrease detector pitch. C. Increase scan time. D. Decrease milliamperage (mA).

D. Decrease milliamperage (mA). Reducing the mAs lowers the overall quantity of x-radiation produced and will in turn reduce the radiation dose to the patient.

Which of the following would increase the signal-to-noise ratio of a CT image? A. Decreased aperture size B. Decreased mAs C. Increased filtration D. Increased aperture size

D. Increased aperture size The signal-to-noise ratio of a 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 (beam width) would accomplish that.

Which of the following CT image archival media is capable of storing the greatest amount of information? A. Magnetic tape B. 5-inch floppy disk C. 3-inch floppy disk D. Magnetic optical disk

D. Magnetic optical disk With storage capabilities of several gigabytes (GB) of digital information, the magnetic optical disk has the highest storage capacity of the archival media listed.

Which of the following technical parameters would greatly improve the quality of CT studies requiring multiplanar reformation (MPR) images? A. Noncontiguous scans B. Wide section thicknesses C. Contiguous scans with wide section thickness D. Overlapping scans with narrow sections

D. Overlapping scans with narrow sections The quality of multiplanar reformats can be greatly improved with the use of narrow, overlapping sections.

Which of the following formulas may be used to calculate the dimensions of a pixel? A. Pixel size = matrix size/DFOV B. Pixel size = DFOV ✕ matrix size C. Pixel size = slice thickness/matrix size D. Pixel size = DFOV/matrix size

D. Pixel size = DFOV/matrix size The dimensions of a pixel may be calculated by dividing the DFOV by the matrix size. Keep in mind that the pixel is a square with four equal sides and the dimension of one side is usually given in millimeters (mm). The area of the pixel is calculated by squaring the dimension, and the units are then adjusted to square millimeters (mm2).

__________ is the term used to describe the ability of a PACS to transmit data from the imaging facility to an off-site location. A. Encryption B. DICOM C. HL7 D. Teleradiology

D. Teleradiology Teleradiology refers to the ability of a PACS to transmit image data across a local area network (LAN) or wide area network (WAN) from the imaging facility to an off-site location.

The process of scanning a phantom device of known density to improve the accuracy of CT attenuation measurement may be referred to as: A. Hounsfield correction. B. linearity. C. uniformity. D. calibration

D. calibration During the process of calibration, CT acquisitions are made of a phantom of a known density (i.e., water) or an empty gantry field (air). The CT system makes corrections of the data acquired, ensuring that the calculations of linear attenuation and Hounsfield values during clinical procedures are accurate.

The primary advantage of an isotropic MDCT data set is: A. reduced scan time. B. increased signal-to-noise ratio (SNR). C. improved raw data convolution. D. high-quality multiplanar reformations (MPRs).

D. high-quality multiplanar reformations (MPRs). An isotropic volumetric data set yields high-quality images with equal resolution in any reconstructed plane. Reconstruction of isotropic, overlapping, thin-section MDCT images greatly reduces the "step" artifact that can negatively affect the quality of MPR and 3-D CT images.

Increasing the detector pitch during a spiral CT examination adversely affects the spatial resolution along which of the following? A. Axial plane B. x-axis C. y-axis D. z-axis

D. z-axis The spatial resolution along the z-axis is decreased when the detector pitch is increased. An increase in detector pitch causes less information to be acquired for each section reconstructed. The section sensitivity profile (SSP) is broadened, and the spatial resolution is reduced.