Mosby Online

Question 1

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

Answer 1

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.

Question 2

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.

Answer 2

C. Compton effect.

Question 3

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

Answer 3

Tube and detectors.

Question 4

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. Homogenous radiation.
B. Unattenuated radiaton.
C. Monochromatic radiation.
D. Remnant radiation.

Answer 4

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.

Question 5

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

Answer 5

Hounsfield and Cormack.

Question 6

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

Answer 6

Compton effect.

Question 7

The types of detectors used in Computed Tomography are: (pick 1 or more)
1. Gas ionization.
2. Stimulable phosphor.
3. Scintillation.

Answer 7

1 and 3 only.

Question 8

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.

Answer 8

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.

Question 9

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

Answer 9

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.

Question 10

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.

Answer 10

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.

Question 11

The approximate inherent filtration of the CT x-ray amounts to an aluminium equivalent of:
A. 1.2 mm.
B. 3.0 mm.
C. 5.8 mm.
D. 10.0 mm.

Answer 11

B. 3.0 mm.

Question 12

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.

Answer 12

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.

Question 13

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.

Answer 13

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.

Question 14

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

Answer 14

Total collimation.

Question 15

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

Answer 15

Z-axis.

Question 16

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

Answer 16

Detector configuration.

Question 17

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

Answer 17

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

Question 18

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

Answer 18

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.

Question 19

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.

Answer 19

D. Calibration.

Question 20

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.

Answer 20

D. Overlapping scans with narrow sections.

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

Question 21

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

Answer 21

Radon.

Question 22

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.

Answer 22

B. Iterative technique.

Question 23

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.

Answer 23

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.

Question 24

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.

Answer 24

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.

Question 25

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.

Answer 25

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.

Question 26

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.

Answer 26

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.

Question 27

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.

Answer 27

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.

Question 28

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.

Answer 28

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.

Question 29

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.

Answer 29

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).

Question 30

The primary advantage of an isotropic MDCT data set is:
A. Reduced scan time.
B. Increased SNR.
C. Improved raw data convolution.
D. High-quality multiplanar reformations (MPRs).

Answer 30

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.

Question 31

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

Answer 31

A. 180-degree interpolation

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.

Question 32

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.

Answer 32

A. Thresholding.

Question 33

The following formula is used to calculate the linear attenuation coefficient:
I = I0e–μx
The symbol “x” identifies:

Answer 33

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.

Question 34

A voxel may be defined as:

Answer 34

A volume element.

Question 35

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

Answer 35

Ray.

Question 36

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.

Answer 36

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.

Question 37

The average CT value for blood is approximately:

Answer 37

+45 HU.

Question 38

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

Answer 38

B. Pixel size & 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.

Question 39

Formula to calculate the dimensions of a pixel?

Answer 39

Pixel size = DFOV/matrix size.

Question 40

A voxel’s dimension may be decreased by which of the following?
A. decreasing the

Answer 40

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.

Question 41

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%

Answer 41

A. 0.1%

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

Question 42

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

Answer 42

range of CT numbers (pixels) to be displayed.

Question 43

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

Answer 43

Section Width.

Question 44

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

Answer 44

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 (212 = 4096).

Question 45

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

Answer 45

-1024 HU and +3071 HU.

Question 46

The quantity of radiation dose received by the pt from a series of CT scans is referred to as the:

Answer 46

MSAD.

The multiple-scan average dose (MSAD) is used to quantify the amount of exposure a patient receives from a series of CT scans. It is calculated from the computed tomography dose index (CTDI) through a series of equations. The quantity of radiation per scan is measured with an ionization chamber.

Question 47

Which of the following technical characteristics of a CT system does NOT directly affect patient dose?
A. Source-to-detector distance.
B. Filtration.
C. Detector efficiency.
D. Filament size.

Answer 47

D. Filament Size.

Although they are beyond the control of the operator, the source-to-detector distance, filtration, and detector efficiency of a CT system all directly affect patient dose. The selected filament size does have geometric ramifications with regard to spatial resolution. A smaller filament results in improved spatial resolution. Patient dose is not affected by filament selection, provided that the mAs value remains constant.

Question 48

Which of the following statements most accurately describes the relationship between slice sensitivity profile (SSP) and dose profile?
A. The SSP is always wider than the dose profile.
B. The SSP is equal in width to the dose profile.
C. The dose profile is always wider than the SSP.
D. The SSP has no effect on the dose profile.

Answer 48

C. The dose profile is always wider than the SSP.

The term slice sensitivity profile (SSP) may be used to describe the reconstructed CT section. However, the section of tissue exposed to ionizing radiation, or dose profile, is greater in width than the SSP.

Question 49

The approximate radiation dose measured during a scan of a 100-mm-long pencil ionization chamber is termed the:

Answer 49

CTDI100

CTDI100 is a fixed measurement taken with a 100-mm-long pencil ionization chamber inserted into an acrylic phantom. The CTDI100 standardizes the dose measurement from a single axial scan and can be used to compare doses from different scan techniques.

Question 50

The calculation of the average cumulative dose to each section within an acquisition of multiple sections is termed the:

Answer 50

MSAD.

The multiple-scan average dose (MSAD) is a calculation of the average cumulative dose to each section within the center of a scan consisting of multiple sections. Because of the contribution of scatter along the z-axis length of an acquisition, the MSAD for each section is higher than the dose from an acquisition of a single section.