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CT Image Quality
Study Questions
| Question | Answer |
|---|---|
| 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
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