CT Production 2

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:

+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 window levels was most likely used to display the image shown? (bone shoulder image)

+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

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:

+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. T -his 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:

+45 HU

The typical range for possible pixel values in a modern CT system is between:

-1024 HU and +3071 HU

Which of the following azimuth settings was used to produce an AP localizer image?

0 degrees

Two adjacent pixels are measured to have a difference of 1 HU. This amounts to a tissue density difference of approximately:

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 (dental implants) would be:

0.29 mm × 0.29 mm × 1.0 mm

A CT scanner with a limiting resolution of 15 lp/cm can resolve an object as small as:

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 512^2 matrix and a display field of view of 17 cm. What is the linear dimension of each pixel?

0.33 mm The dimension of a pixel may be calculated by FOV/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

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:

1

Which of the following components of CT image quality is/are being evaluated? See Fig. (pic of water/daily tester phantom) 1. Linearity 2. Section width 3. Spatial resolution

1 and 2

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)

1 and 2

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.

1 and 2 -Check CT number calibration -Examine noise levels (standard deviation) of a water phantom.

Soft copies of CT images are stored on which of the following media? 1. Hard drives 2. Magnetic optical disk 3. Laser film

1 and 2 hard drive and magnetic optical disk

The types of detectors used in computed tomography are: 1. Gas ionization 2. Stimulable phosphor 3. Scintillation

1 and 3

Which of the following may be used for the archival storage of CT images? 1. CD-ROM 2. VHS tape 3. Magnetic optical disk

1 and 3

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

1 and 3 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.

1 and 3 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)

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

1 only

Important characteristics of a quality control program in CT include: 1. Consistent testing frequency 2. Prompt corrective actions when necessary 3. Accurate record-keeping

1, 2, and 3

Which of the following components of CT image quality may be controlled by the technologist? 1. Spatial resolution 2. Contrast resolution 3. Noise

1, 2, and 3

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

1, 2, and 3

Which of the following technical factors exhibit an effect upon longitudinal spatial resolution? 1. Detector collimation 2. Detector pitch 3. Interpolation algorithm

1, 2, and 3

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?

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

Each 1.0% in contrast between adjacent objects amounts to a difference in pixel value of approximately:

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 matrices would provide the greatest spatial resolution?

1024 x 1024

Which of the following section widths would cause the greatest amount of partial volume averaging?

10mm

Which of the following mathematical techniques is used for the reconstruction of volumetric MDCT images?

180-degree interpolation (180LI)

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

1979

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.

2 and 3

Which of the following steps could be taken to reduce the artifact? See Fig.(dental implants) 1. Reduce section thickness. 2. Angle gantry around fillings. 3. Decrease kVp.

2 only

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?

2.15 mm3

Which of the following acquisitions may be characterized as overlapping?

2.5-mm sections reconstructed every 1.25 mm

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?

20 mm

What is the display field of view used for a 3202 matrix image with a pixel dimension of 0.75 mm ✕ 0.75 mm?

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.

The approximate inherent filtration of the CT x-ray amounts to an aluminum equivalent of:

3 mm

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.

3 only

In a modern CT system, the total number of possible Hounsfield values that may be assigned to any one pixel is approximately:

4096

Which of the following acquisitions may be characterized as noncontiguous?

5.0-mm sections reconstructed every 7.5 mm

The human eye is capable of differentiating approximately __________ shades of gray.

60 to 80

CT scanners use between 120 and 140 kVp, yielding x-ray energies with averages between what keV

70 and 80 keV

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

80✕80

Which of the following manipulations involves the use of image data?

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

Maximum intensity projections (MIPs) are commonly used during specialized studies such as CT angiography. Describe how it is constructed?

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 reduces the scatter radiation reaching the detectors?

Both a and c

How are segmented images constructed?

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

Which of the following technical factor adjustments will result in a reduction in patient radiation dose?

Decrease milliamperage (mA)

A voxel's dimension may be decreased by which of the following?

Decreasing the section width

The first-generation prototype CT scanner designed by

Dr. Godfrey Hounsfield

Which of the following is not an iterative method of CT image reconstruction?

Fourier transform

In 1979, the scientists __________ and __________ shared the Nobel Prize for their research in computed tomography.

Hounsfield and Cormack

Which of the following adjustments in technical factors will result in an increase in spatial resolution?

Increase in sampling frequency

Which of the following would increase the signal-to-noise ratio of a CT image?

Increased aperture size

Which of the following types of image reconstruction was used in the first prototype CT scanner?

Iterative technique

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):

MIP image

Which of the following CT image archival media is capable of storing the greatest amount of information?

Magnetic optical disk

Which of the following archival media is capable of storing the largest number of CT images?

Optical disk

Which of the following technical parameters would greatly improve the quality of CT studies requiring multiplanar reformation (MPR) images?

Overlapping scans with narrow sections

Which of the following formulas may be used to calculate the dimensions of a pixel?

Pixel size = DFOV/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).

The dimensions of a voxel may be calculated as the product of which of the following?

Pixel size and section width

In 1917, Austrian mathematician __________ proved that it was possible to reconstruct a three-dimensional object from the infinite set of all of its projections.

Radon

Which of the following technical adjustments would result in a decrease in the signal-to-noise ratio (SNR)?

Reducing the section width

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?

Standard deviation

__________ is the term used to describe the ability of a PACS to transmit data from the imaging facility to an off-site location.

Teleradiology

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?

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.

The term beam hardening is used to describe which of the following physical phenomena?

The increase in average photon energy of a heterogeneous x-ray beam -The heterogeneous beam undergoes an increase in average photon energy as it passes through the patient and the lower-energy photons are absorbed.

For single-slice CT (SSCT) systems, which of the following statements regarding retrospective image reconstruction is FALSE?

The slice thickness and SFOV may be changed.

Which of the following is not used to archive CT images?

VHS tape

Which of the following types of gas is commonly used for gas ionization CT detectors?

Xenon

What if uniformity?

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

The following formula is used to calculate the linear attenuation coefficient:I = I0e-μxThe symbol "x" identifies:

absorber thickness

What is a 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.

As a solid-state CT detector measures transmitted radiation, it emits a proportional response in the form of a(n):

analog signal

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):

artifact

Phantom measurement of the uniformity of a CT system is performed primarily to assess the negative effects of:

beam hardening

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

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

The type of compensating filter used at the x-ray tube of a CT scanner is called a

bow-tie filter

Cupping artifacts most commonly occur in the:

brain

The process of scanning a phantom device of known density to improve the accuracy of CT attenuation measurement may be referred to as:

calibration

The computer technique used to reduce the size and storage requirements of digital CT image data is called:

compression

Which of the following is the primary interaction between x-ray photons and tissue during computed tomography?

compton effect -CT scanners use between 120 and 140 kVp, yielding x-ray energies with averages between 70 and 80 keV

The interaction between x-ray and matter that is responsible for the production of the scatter radiation absorbed by the patient and detectors is:

compton interaction

The term __________ describes the ability of a CT scanner to differentiate objects with minimal differences in attenuation coefficients.

contrast resolution -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:

cross-field uniformity 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.

Which of the following would increase the noise apparent on a CT image?

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

Which of the following will serve to decrease the noise of a CT image?

decrease in matrix size

Which of the following is capable of causing an edge gradient artifact?

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.

Which of the following components of the CT system is the most common cause of ring artifacts on the reconstructed CT image?

detector

The number, length, and organization of the individual detector elements in an MSCT system are referred to as the:

detector configuration

A straight line appearing vertically on the scanogram (pilot) of a fourth-generation CT scanner is an artifact most likely caused by:

detector malfunction

For a given CT acquisition, the calculated mAs applied to each reconstructed slice may be referred to as the:

effective mAs -the calculated mAs per acquired slice

The full width at half maximum (FWHM) of a slice sensitivity profile diagram indicates the:

effective section width

Which of the following terms accurately describes the type of x-ray beam used in a third-generation CT scanner?

fan beam

The spatial resolution of a CT image depends on several factors, including what 5?

focal spot size detector size and spacing field of view pixel dimension detector sampling rate

Areas of a CT image containing abrupt changes in tissue density are electronically represented by:

high spatial frequencies.

The primary advantage of an isotropic MDCT data set is:

high-quality multiplanar reformations (MPRs).

The type of MSCT detector array that contains midline narrow elements flanked by wider detectors is called a(n):

hybrid array

Which of the following technical changes may increase the partial volume effect present on a spiral CT scan?

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 factors has no measurable effect on spatial resolution?

kVp

Which of the following is used to archive a hard copy of a CT image?

laser film

The ability of an object to attenuate the x-ray beam is assigned a value known as the:

linear attenuation coefficient

The broadening of the slice sensitivity profile inherent to helical CT acquisition is said to negatively affect the:

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

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 spatial resolution of a CT scanner is usually given in the unit of measure:

lp/cm

The spatial resolution of a CT scanner is often measured using the MTF of the system, which is an acronym for:

modulation transfer function

A voxel whose attenuation coefficient is less than that of water is assigned a pixel value with a(n) __________ CT number.

negative

The computerized communication system used by a PACS to transmit data is referred to as a:

network

A CT image of a homogeneous material contains variations in CT number from pixel to pixel. This image is said to have:

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 largest deterrent of a CT scanner's contrast resolution is:

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.

What is 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

First generation CT scanners use what type of beam?

pencil beam -used with only two detectors

A pixel may be defined as:

picture element

The component of the DAS responsible for strengthening the signal emitted from a detector is termed the:

preamplifier

Which of the following types of image noise can be most easily reduced by the CT technologist?

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

When one is choosing a window to display a CT image, the width defines the:

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 portion of the primary beam interacting with a single detector is known as a:

ray

The volume of a voxel may be calculated by multiplying the pixel dimension (mm2) by the:

section width

CT images that have been reconstructed from a portion of the data acquisition process in the hopes of reducing patient motion artifacts are called:

segmented images segmentation 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

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

sensitivity contrast resolution A CT scanner with poor contrast resolution or low sensitivity has difficulty separating tissues whose linear attenuation coefficients are nearly equal

The iterative methods of CT image reconstruction include

simultaneous reconstruction, ray-by-ray correction, and point-by-point correction.

The degree of CT section broadening that occurs along the z-axis during volumetric data acquisition is represented graphically by the:

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

The ability of a CT scanner to image a small high-density object is controlled by the __________ of the scanner.

spatial resolution

The Digital Imaging and Communications in Medicine (DICOM) protocol was developed to:

standardize the networking and storage of digital images.

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:

surface rendering

The level is set at

the average Hounsfield value for the tissue of interest within a range determined by the window width

The process by which electrons are produced at the cathode of a CT x-ray tube is known as:

thermionic emission

The technique that allows the user to select the range of pixel values used in a 3-D CT reformation is termed:

thresholding

In multislice CT (MSCT), the beam pitch is equal to the table feed per rotation divided by the:

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.

TRUE or FALSE Narrow section widths tend to decrease the partial volume effect?

true

The assignment of different generations to CT scanners is based on the configuration of the:

tube and detectors

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:

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

The ability of a CT system to maintain consistent Hounsfield values across the entire image of a homogeneous object is termed:

uniformity

During a CT scan, each sample of ray sum measurements made by the data acquisition system (DAS) is called a:

view

A voxel may be defined as which of the following?

volume element

The CT number is a relative value that standardizes the attenuation value (μ) of a tissue by comparing it with the attenuation of:

water

What causes a cupping artifact?

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.

Which of the following is used in gas ionization CT detectors?

xenon

In an MSCT system, the detector array is composed of multiple rows of individual detector elements along the:

z-axis

Which of the following corresponds to the longitudinal dimension of the CT image voxel?

z-axis

Increasing the detector pitch during a spiral CT examination adversely affects the spatial resolution along which of the following?

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.

The Hounsfield value of a pixel is directly related to which of the following?

μ of H2O