CT Image Appearance Flashcards
The resolution of a CT scanner is often expressed in terms of…
“Line pairs”
How can you determine spatial resolution?
Centimeter divided by lines (in a line pair)
What can we adjust to affect resolution?
Pixel size
Slice thickness
Reconstruction filter
What factor would make an image appear more noisy?
Insufficient number of photons contributing to the pixels
What are some factors that influence parameter selection?
Contrast
Resolution
Noise
Dose
Why would applying a noise-reducing algorithm during image reconstruction be a preferred method?
Can minimize the dose without compromising the image quality
What determines the number of x-ray photons produced by the CT x-ray tube?
mA
What happens when you increase the mAs / kVp?
Dose increases while noise decreases
What determines the maximum energy of the x-ray photons produced by the CT x-ray tube?
kVp
What does it mean when you increase the kVp?
Increases the photon’s ability to penetrate the tissues of the patient’s body
Which parameter controls the contrast in a CT image?
kVp
On single-row detector scanners, slice thickness is partially defined by:
The size of the focal spot in the CT x-ray tube
On single-row detectors, the acquired slice thickness is equal to:
The collimated x-ray beam thickness (as well as the final image thickness)
What does the acquired slice thickness indicate?
Which rows of detectors are being used to measure the photons
What are the two slice thickness parameters?
Acquired slice thickness
Reconstructive (“effective”) slick thickness
What does the reconstructive (“effective”) slice thickness indicate?
The thickness of the final image
On multi-row detector scanners, the final slice thickness must be equal to or greater than:
The acquired slice thickness
What happens when you increase the slice thickness?
Decreases noise
Increases amount of anatomy being covered
The result of a decrease in resolution due to slices that are too thick:
Partial voluming
If you increase the slice thickness while covering the same anatomical region, what happens to the radiation dose to the patient?
May decrease
How can you achieve contiguous slices?
The table increment must be equal to the slice volume
What does selecting a value for the pitch specify?
How compressed or stretched the path of the x-ray beam takes on a helical scan
On a single-row detector, what is the pitch equal to?
Movement of the patient table in one gantry rotation divided by the slick thickness
On a multi-row detector, what is the pitch equal to?
Movement of the patient table in one gantry divided by total slice volume or thickness of a single slice
A pitch that is increased / decreased results in a _____ / _____ helix
Stretched / Compressed
What are some reasons to increasing the pitch?
Greater anatomical coverage in less time
Faster scan for trauma patients
Appropriate contrast timing during a CTA study
Decrease dose to the patient
How is the radiation dose to the patient related to the pitch?
Reducing the pitch increases dose to the patient
Reconstruction interval may be modified and the new value may be applied through:
Retrospective reconstruction
If the reconstruction interval is equal to the reconstructed slice thickness, the reconstructed slice thickness will be:
Contiguous
If the reconstruction interval is less than the reconstructed slice thickness, the reconstructed slice thickness will be:
Overlapped
Increasing the reconstruction FOV will make the anatomy appear:
Smaller
Increasing the reconstruction FOV will make the image appear:
Less noisy
The size of a single pixel is equal to:
The reconstruction FOV divided by the number of pixels in the image matrix
What happens when the image matrix increases?
Yield smaller pixels (better resolution)
Increases noise
How is the degree of beam hardening determined?
By dense structures and travelling further through the tissues of the body
Where may you might see beam hardening manifesting itself as a cupping artifact?
In the supratentorium of the brain
What are ways in which you can minimize beam hardening artifacts?
Use correction algorithms
Using thin slices
Don’t scan through very dense contrast media
What can be the result of a partial volume artifact?
Blurriness
Incorrect CT numbers
Missed pathology / false positives
How can you minimize partial volume artifacts?
Using thinner slices
A high window level will make the image appear:
Darker allowing more dense structures to be evaluated
A low window level will make the image appear:
Brighter allowing less dense structures to be evaluated
A display filter is applied directly to the _____ data.
Digital image
Why would you not want to apply a display filter when taking ROI measurements?
Can vary the pixel values
False ROI readings
Magnification is a post-processing technique applied to the ____ data.
Image
