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CT Image Quality

Study Questions

QuestionAnswer
Typically, tissues with differences in density of less than ______% can be differentiated with CT. 0.5%
What is the required difference in density in order for tissues to be differentiated with conventional radiography? 10%
How is spatial resolution measured? line pairs per cm
In a spatial resolution test for CT, what is one hole and one space referred to as? line pair
What does a resolution test consist of? scanning a water filled phantom and determining the smallest line pair that can be resolved.
The black stripes in a CT spatial resolution phantom represent the ______. holes
What appears in a CT image as a point-to-point fluctuation in the CT number within a uniform material? noise
The CT number scale is established relative to the attenuation of what? water
What is the CT Number/Hounsfield Unit for water? 0
What is the CT Number/Hounsfield Unit for air? -1000
What is the CT Number/Hounsfield Unit for fat? approximately -80
What is the CT Number/Hounsfield Unit range for soft tissue? approximately 30 to 80
Any measurements above _______ units generally indicates the presence of calcification. 100
Approximately what range does Congealed blood fall into? 56 to 76 HU
Approximately where does blood fall on the CT Number/Hounsfield scale? 12 HU
What is the only reason a technologist might consider changing the kVp for a given exam? if the patient is very large or very small
What are the two factors that determine slice thickness on a single-row detector scanner? 1. focal spot size 2. pre-patient collimation
What is another name for reconstructed slice thickness? effective slice thickness
The _____ indicates which rows of detectors are being used to measure the photons that pass completely threw the patients body. acquired slice thickness
What do we call the thickness of the final image? reconstructed or effective slice thickness
The use of thicker slices reduces the resolution along the direction perpendicular to the __ direction. z-direction
What do we call the effect that results from the from the mathematical averaging of several millimeters of varying density tissues, which is displayed as a single shade of gray on the resulting image? partial voluming
To get contiguous slices, the table increment must be ___ to the slice thickness. equal
If the pitch is increased to more than 2 (i.e., in one rotation the patient table moves more than twice the total slice volume) ____ artifacts will occur. helical
What do helical artifacts look like? swirls or windmills
On single-row detector scanners, as the pitch is increased, there is an unfortunate increase in the ____. effective slice thickness
If the reconstructed interval is equal to the slice thickness, the reconstructed slice will be ___. contiguous
If the reconstruction interval is less than the slice thickness, the reconstructed slices will ___. overlap
What determines the total area over which projection data is collected? scan field of view (SFOV)
What determines the number of detectors collecting data for a particular scan? SFOV
Minimizing the noise with a large reconstruction FOV is attained only by compromising the image’s _____. spatial resolution
How many shades of gray can an operators console display? 256
How many shades of gray can the human eye distinguish? about 20 shades of gray
What controls how “contrasty” the image appears? window width
What pixel level (color) is assigned to any CT numbers above the range specified by the window width? 255 (white)
What pixel level (color) is assigned to any CT numbers below the range specified by the window width? 0 (black)
When is a wide window width generally used? when viewing structures with a high degree of contrast, such as in the lungs. (Stark differences in density of tissues)
When is a narrow window width useful? when visualizing structures exhibiting subtle inherent contrast, such as with the soft tissues of the brain. (Similar differences in density)
What does the window level control? brightness of the image; specifically, the window level sets the center CT number displayed on the monitor
What are the two most frequently used standards for daily quality assurance assessment? 1. CT Number Calibration Test 2. CT Number Standard Deviation Test
What are the two parameters evaluated during the CT number calibration test/CT number standard deviation test? the average value of the CT numbers and the standard deviation of the CT numbers within a region of interest.
What are the acceptable limits for the CT Number Calibration Test? -3 to +3
What are the acceptable limits for CT Number Standard Deviation? the Standard Deviation of water must not exceed a level specified by the manufacturer.
Created by: sspatel