CT Flashcards
What does the 3rd generation CT mean?
The detector and x-ray unit spin around the patient in synchrony
What is the collimator width in a single slice machine?
Table travel in one 360 degree rotation/Collimator width
What is the collimator width in a multiple slice machine?
Detector pitch?
Detector pitch/number of detectors
Detector pitch = table movement/Detector width
What is the widow width and level for:
ST, bone, lung, brain
ST: 400W and 40L
Bone: 2500W and 400L
Lung: 1500W and -600L
Brain: 80W and 40L
What are the HUs for:
Air, lung, fat, brain white/gray, Muscle, bone, fluids, clotted blood, blood
- Air = -1000
- Lung = -800 to -300
- Fat = -150 to -60
- Brain (white) = 20-30
- Brain (gray) = 35-45
- Muscle = 40-60
- Bone = 500-3000
- Blood clot = 45-70
- Blood = 30-45
What artifact does the 4th generation CT scanners?
Ring artifact
What does a bowtie filter do?
Filters the peripharal beam more to conpensate for uneven attenuation.
How do you calculate pixel size?
FOV/Matrix
What is overscanning?
Pitch less than 1
HUs are dependent/based on what?
Attenuation of water… therefore changing kVp and filtration can change HU values
What are the ways in which the CT makes a projection from the raw data?
- Back projection - old way
- Filter back projection - newer way
- Iterative reconstruction
- Forwarded information is compared to actual information
- Corrects for noise and losers dose.
What is a kenrel/mathematical filter?
Reconstruction algorithm
Sharp, bone = increase spatial resolution but increased noise
Smooth = low noise but reduced spatial res.
What is the advange of Axial scanning?
Better spatial res
Less partial volume artifact
If you increase beam width what happens?
Reduce scan time
Reduce motion
Increase volume averaging
Dose not change dose
What increases SNR on CT?
- Higher mA
- Twice the x-ray influx = twice the signal but Square root(2) increase in noise or 1.4
- So…2/1.4 = 1.4 or a 40% increase in SNR
- Higher kVp
- Longer rotation time
- Larger slice thickness
- Larger pixel
- Decreased pitch
- Noise reduction filter (ST…not bone)
What increases Spatial resolution in CT?
- Smaller focal spot
- Smaller detector aperture size in the cranial caudal direction
- More projections
- Thinner slices
- Small DFOV
- Less pitch
- Sharp filter (bone)
What is overranging?
Starting 1/2 the space beyond scan length and after (only helical)… this is due to the pitch and how you might miss some stuff.
Beam hardening looks like what? Fixing it?
- Cupping
- Dark bands/Streaks
Fixing it:
- Pre-hardening
- Calibration Correction
- Software
- Avoidance
What is the difference between beam hardening and photon starvation and Metal artifacts?
The all can have streaking.
Beam hardening is when the lower energy beam is attenuated
Photon starvation is when most of the beam is attenuated
Metal artifact is when most of the beam is attenuated and other artifacts like density range happen
How do you correct metal artifact?
Increase kVp
Thinner slices
Software
Which metals tend to have more artifact?
High Z metals (Iron and platium)
Compared to low Z (titanium)
