CT REGISTRY- STUDY THIS AND
YOU'LL PASS! (EXCLUDING ANATOMY)
QUESTIONS AND ANSWERS WITH
SOLUTIONS 2024
how do you reduce beam hardening - ANSWER 1. Increase dose
2. Reduce collimation--Reduce slice thickness
3. Increase window width
At what level does the abdominal aortal bifurcate? - ANSWER L4
What is the typical scan delay after injection of contrast media for studies of the liver? - ANSWER 30-45
seconds
What window settings provide the best tissue differentiation within the liver? - ANSWER window width=
140, level= 60
What is the window settings that provide the best bone window for the pelvis? - ANSWER window
width=2000, window level=350
What mAs is typically used in routine CT exams of the abdomen? - ANSWER 200-300 mAs
What is the result of the bifurcation of the abdominal aorta? - ANSWER The right and left common iliac
arteries
What is the anode target angle? - ANSWER 12 degrees
At what level do the common carotids bifurcate into the internal and external carotid arteries? -
ANSWER C3-C4
The floor of the orbit is formed by the __________ bones. - ANSWER maxillary and zygomatic
,The lateral wall of the orbit is formed by the ___________ bones. - ANSWER zygomatic and sphenoid
The medial wall of the orbit is formed by the _________ bones. - ANSWER ethmoid and lacrimal bones
how do you calculate effective mAs? - ANSWER mAs/pitch
define window level and window width - ANSWER LEVEL: a chosen midpoint in the grayscale
WIDTH: number of grayscale values above and below the level.
- The width is DIVIDED in HALF and distributed above and below the level.
- Anything above the window is white
- Anything below the window is black
The wider the window the more grayscale values, the lower the contrast!
example:
Level 100, Width 300
White = >+250
Black = <-50
Level 100, Width 200
White = >+200
Black = <0
Level 100, Width 150
White = >+175
Black = <25
,factors that affect spatial and contrast resolution - ANSWER Spatial resolution:
1) focal spot
2) detector width (aperture)
3) reconstruction algorithm - bone>soft tissue
4) slice thickness - thinner is better
5) pixel/FOV/matrix
6) pitch - decreased pitch means no gaps
7) nyquist limitations
Contrast resolution: CNR
1) energy of X-rays - kVp - ↑ kVp ↓ contrast
2) number of X-rays - mA - ↑ mA ↓ mottle
3) slice thickness - thicker is better
4) reconstruction method - iterative > filtered
5) reconstruction algorthith - soft tissue>bone
Give window levels for brain, lung, abdomen, bone - ANSWER Brain: W80,L+40
Lung: W1500, L-400
Abdomen: W400, L+50
Bone: W1600, L+500
What is beam hardening?
what are the two types of beam hardening artifact?
how do you compensate for it? - ANSWER 1) as the X-ray beam passes through an object, lower energy
photons are removed (like filtration!) leaving a "harder beam" of high energy photons. This causes two
artifacts:
2) Cupping: the center of a round object is darker than the periphery due to hardening of the beam 360
degrees around the object. happens in the head
, 3) Streak: these are dark bands that occur in between two dense objects - nearly all the photons are
removed in a line between the two objects
4) fixes:
A) Filtration: pre-hardening the beam to remove low energy photons
B) Calibration: using a phantom to set a compensated mA/kVp
C) Correct software: iterative reconstruction may help
D) Avoidance: tilting the gantry of positioning the patient to avoid areas that may cause hardening.
ring artifact - ANSWER Calibration error or defective detector cause consistently erroneous reading at
each angular position, resulting in a circular artifact
is the focal spot large or small? - ANSWER large 0.6-1.2 - so as not to overheat the anode from such a
high mA
Which of the following will result in an increased voxel size: B increased slice thickness - ANSWER
A)decreased reconstruction FOV B) increased slice thickness C) increased matrix D) none of these
If a tissue with a CT number of +300 appears white on the image, which of the following are the window
width and window level selected: A)window level =0, window width =500 B)window level= +300,
window width= 500 C)window level =+100, window width =1000 D)none of these - ANSWER A) window
level =0, window width =500
What is the volume of a voxel if a slice thickness of 3 mm, a 512x512 matrix & a reconstruction field of
view of 25.6cm are used A) 0.75mm³ B)1.5mm³ C) 6.0mm³ D)none of these - ANSWER A. Solve this by
finding the pixel size first FOV/matrix size ... that is 0.5 ... now square that 0.25 .... multiply that by slice
thickness and you get 0.75
Which of the following parameters is responsible for partial volume averaging: A) matrix size B) kVp
C)slice thickness D) patient dose - ANSWER C
Voxel equation - ANSWER pixel area x section thickness
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