CT Image Quality
What is the expected result of a CT Number Calibration Test performed on a water phantom?
0+or-3
The smallest object that can be resolved if a reconstruction FOV of 24 cm and a 512 x 512 matrix is how many mm?
0.47
If a reconstruction FOV of 32 cm and a 512 x 512 matrix are used, the pixel size is ________ mm.
0.625 x 0.625
What value for pitch would stretch the x-ray helix the most? a. 1.5 b. 1.0 c. 0.8 d. 0.5
1.5
If the radiologist requires 2 mm thick contiguous slices through the pituitary, and 2.4 cm of coverage, how many slices are necessary?
12
On a conventional, or serial, CT scan if we want 5mm slices with 1mm of overlap, the table increment should be how many mm?
4
If the slice thickness is changed from 5 mm to 7 mm and all other parameters are held constant, what percent more photons will be used in making the slice?
40
What is the conversion equation from CT number to Hounsfield unit?
CT number = Hounsfield unit
What affects the CT number of water?
No imaging parameter
For a given distance of coverage, which of the following is TRUE? 1. as reconstruction FOV increases, patient dose increases 2. as pitch increases, patient dose decreases 3. as slice thickness increases, patient dose decreases
as pitch increases, patient dose decreases and as slice thickness increases, patient dose decreases
CT numbers _________ would appear black if a window width of 400 and a window level of -100 are used.
below -300
Which of the following has the highest patient dose assuming all other parameters are the same? a. mA=200, rotation time=2 seconds b. mA=200, rotation time=1 second c. mA=150, rotation time=3 seconds d. mA=400, rotation time=1 second
c. mA=150, rotation time= 3 seconds
A CT Number Calibration Test should be performed how frequently?
daily
What are the possible results of partial voluming?
decreased sharpness of edges and misleading CT numbers
What is a reason to increase the pitch?
greater volume of anatomy can be covered in a given time and a given volume of anatomy can be covered faster
If a thin slice is used, what would help reduce the apparent image noise?
increase mAs
Which of the following will result in an increased voxel size? 1. decreased reconstruction FOV 2. increased slice thickness 3. increased matrix
increased slice thickness
_______ the window _______ will make the image appear darker so that bony structures can be evaluated.
increasing, level
What determines how penetrating the x-ray beam will be?
kVp
What parameter affects the contrast of a CT image?
kVp
Compared to a CT scan protocol without iterative reconstruction applied, when using iterative reconstruction the scan parameter most likely to be reduced is what?
mAs
Which of the following statements is FALSE? 1. magnification requires the raw data 2. increasing magnification increases resolution 3. magnification decreases the size of displayed anatomy
magnification requires the raw data, increasing magnification increases resolution and magnification decreases the size of displayed anatomy
How many total pixels are in a 1024 x 1024 matrix
more than a million
What can be used to better visualize low contrast resolution?
narrow window width
What determinants of CT image quality is responsible for how grainy or speckled a CT image appears?
noise
On MDCT, the final reconstructed slice thickness is determined by what?
number of rows of detectors and the size of detectors whose attenuation data gets combined
Increasing _______ results in lowering the patient dose.
pitch and table increment
Which of the following does NOT occur when reconstruction FOV is decreased? 1. spatial resolution is increased 2. pixel size is increased 3.noise in the image increases
pixel size is increased
What parameter allows adjacent slices to be overlapped without increasing the dose to the patient?
reconstruction interval
What parameter causes helical slices to be reconstructed so they overlap?
reconstruction interval
Which of the following would be used in a high resolution study of the temporal bones? 1. reduced slice thickness 2. reduced reconstruction FOV 3. sharp reconstruction filter
reduced slice thickness, reduced reconstruction FOV and sharp reconstruction filter
Which of the following is FALSE about the spatial resolution of an image? 1. resolution is dependent on matrix size 2. resolution is dependent on reconstruction FOV 3. resolution is dependent on scanned FOV
resolution is dependent on scanned FOV
What parameter is responsible for partial volume averaging?
slice thickness
The distance that the table travels between slices is called what?
table increment
The ability to scan a moving organ without seeing motion artifacts on the images is an example of an exam performed with high:
temporal resolution
What is NOT directly proportional to mAs?
the mean energy of the x-ray photons leaving the tube
A set of images is reconstruction with a 150 mm reconstruction FOV. Keeping all other parameters constant, a second set of images is reconstruction with a 300 mm reconstruction FOV, and then magnified so that the anatomical structures in the first and second set appear the same size. What is FALSE?
the resolution of the second set of images is better than the first, the apparent noise in the second set of images is more pronounced than the first set and the patient dose of the first set of images is higher than the second set.
Which of the following can be changed retrospectively? 1. the slice thickness from an MDCT helical scan 2. the slice overlap from a single-row detector scanner helical scan 3.the slice thickness from a single-row detector scanner helical scan
the slice thickness from an MDCT helical scan and the slice overlap from a single-row detector scanner helical scan
An increase in what will result in hardening of an x-ray beam?
tissue density, tissue thickness, and kVp
If a calcification with a CT number of +300 appears pure white on an image, which of the following could be the window width and window level selected? 1. window level= 0, window width= 500 2. window level= +300, window width=500 3. window level= +100, window width=1000
window level= 0, window width= 500
