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151. The contrast resolution of a CT scanner is not related to which of the following?
a. focal spot size
b. section width
c. reconstruction algorithm
d. signal-to-noise ratio
a. focal spot size
With multidetector CT (MDCT), detector collimation determines
the width of the reconstructed section. By electronically adjusting
the detector dimension, the operator can control the width of the
x-ray beam contributing to a reconstructed section. Beam
collimation no longer directly controls section width.
152. The artifact present on the lateral borders of the image in the figure most likely represents which of the following? (Figure 6.12)
a. edge gradient
b. out-of-field artifact
c. tube arcing
d. beam hardening
152. B The artifact present in the figure most likely represents an out-offield artifact. This relatively large patient was incorrectly positioned, and a portion of the anatomy lies outside the scanned field of view. This improperly centered anatomy interferes with the reference detectors, thus causing a streak artifact near the unscanned area.
153. The image in the figure was produced using the following parameters: large (full) scan field of view (SFOV); maximum display field of view (DFOV) (48 cm); 200 mA, 120 kVp; soft tissue algorithm. Which of the following technical adjustments would serve to reduce the artifact present? (Figure 6.12)
a. switching to a detail or bone algorithm
b. using a smaller SFOV
c. increasing the mA to 240 and the kVp to 140 d. centering the patient within the SFOV
153. D The out-of-field artifact present in this image could be easily reduced by properly centering the patient within the scan field of view.
154. The average photon energy of the CT x-ray beam can be increased by:
a. increasing mAs
b. increasing filtration
c. increasing collimation
d. all of the above
Increases in filtration cause a greater amount of low-energy x-ray photons to be absorbed, thereby increasing the average photon energy of the beam. Higher mAs values increase the intensity of the beam but do not affect average photon energy.
155. Which of the following is a solid-state device used to record the light flashes given off by a scintillation crystal?
a. photomultiplier tube
d. input phosphor
155. C Scintillation crystals are used in cooperation with photodiodes in a scintillation-type CT detector. The photodiode is a solid-state device that absorbs the light flashes given off by the crystal. The photodiode then emits an electrical signal in response to this light.
156. Automatic tube current modulation (ATCM) software is used in current multidetector CT (MDCT) systems to control: 390
a. patient radiation dose
b. cardiac motion
c. timing of the contrast agent bolus
d. tissue perfusion assessment
156. A Automatic tube current modulation (ATCM) programs adjust the mA throughout an acquisition to reduce patient radiation dose to a minimum. ATCM automatically alters the applied mA on the basis of a predetermined noise index that is acceptable for appropriate image quality.
157. The type of multidetector CT (MDCT) detector array that contains multiple rows of detector elements, each of the same length, is called a(n):
a. uniform matrix array
b. adaptive array
c. hybrid array
d. stationary array
157. A A uniform matrix array consists of multiple detectors in the longitudinal direction, each with the same dimensions.
158. A CT image is displayed in a window with a level of 0 and a width of 500. Which of the following statements is correct?
a. Pixels with values between 0 HU and 500 HU will appear white.
b. Pixels with values between −250 HU and +250 HU will be assigned shades of gray.
c. Pixels with values greater than +500 HU will be black.
d. Pixels with values less than 0 HU will appear white.
158. B The width of a window determines the range of pixel values that will be assigned shades of gray around a given level. In this example, all pixels within the range of −250 to +250 HU will be assigned shades of gray. Pixels below −250 HU will appear black, and pixels above +250 HU will appear white.
159. Reformatted CT image planes that lie perpendicular to the original plane of acquisition may be described as:
159. B Orthogonal planes are at right angles to each other. Orthogonal reformations are perpendicular to the original plane of data acquisition.
160. Segmenting the data acquisition process into separate components of smaller rotation angles may improve which of the following components of CT image quality?
a. temporal resolution
b. longitudinal spatial resolution
c. contrast resolution
d. in-plane spatial resolution
160. A. The controlling factors of the temporal resolution of a CT system are the gantry rotation speed and reconstruction method. Temporal resolution can be improved by segmenting the data acquisition process into separate components of smaller rotation angles. Single-segment or half-scan acquisitions reconstruct data obtained from half of the rotation time. For example, this type of segmentation would yield a 250-msec temporal resolution from a 0.5-sec gantry rotation time.
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