CT

Question 1

What does the 3rd generation CT mean?

Answer 1

The detector and x-ray unit spin around the patient in synchrony

Question 2

What is the collimator width in a single slice machine?

Answer 2

Table travel in one 360 degree rotation/Collimator width

Question 3

What is the collimator width in a multiple slice machine?

Detector pitch?

Answer 3

Detector pitch/number of detectors

Detector pitch = table movement/Detector width

Question 4

What is the widow width and level for:

ST, bone, lung, brain

Answer 4

ST: 400W and 40L

Bone: 2500W and 400L

Lung: 1500W and -600L

Brain: 80W and 40L

Question 5

What are the HUs for:

Air, lung, fat, brain white/gray, Muscle, bone, fluids, clotted blood, blood

Answer 5

1. Air = -1000
2. Lung = -800 to -300
3. Fat = -150 to -60
4. Brain (white) = 20-30
5. Brain (gray) = 35-45
6. Muscle = 40-60
7. Bone = 500-3000
8. Blood clot = 45-70
9. Blood = 30-45

Question 6

What artifact does the 4th generation CT scanners?

Answer 6

Ring artifact

Question 7

What does a bowtie filter do?

Answer 7

Filters the peripharal beam more to conpensate for uneven attenuation.

Question 8

How do you calculate pixel size?

Answer 8

FOV/Matrix

Question 9

What is overscanning?

Answer 9

Pitch less than 1

Question 10

HUs are dependent/based on what?

Answer 10

Attenuation of water… therefore changing kVp and filtration can change HU values

Question 11

What are the ways in which the CT makes a projection from the raw data?

Answer 11

1. Back projection - old way
2. Filter back projection - newer way
3. Iterative reconstruction
   
   1. Forwarded information is compared to actual information
   2. Corrects for noise and losers dose.

Question 12

What is a kenrel/mathematical filter?

Answer 12

Reconstruction algorithm

Sharp, bone = increase spatial resolution but increased noise

Smooth = low noise but reduced spatial res.

Question 13

What is the advange of Axial scanning?

Answer 13

Better spatial res

Less partial volume artifact

Question 14

If you increase beam width what happens?

Answer 14

Reduce scan time

Reduce motion

Increase volume averaging

Dose not change dose

Question 15

What increases SNR on CT?

Answer 15

1. Higher mA
   
   1. Twice the x-ray influx = twice the signal but Square root(2) increase in noise or 1.4
   2. So…2/1.4 = 1.4 or a 40% increase in SNR
2. Higher kVp
3. Longer rotation time
4. Larger slice thickness
5. Larger pixel
6. Decreased pitch
7. Noise reduction filter (ST…not bone)

Question 16

What increases Spatial resolution in CT?

Answer 16

1. Smaller focal spot
2. Smaller detector aperture size in the cranial caudal direction
3. More projections
4. Thinner slices
5. Small DFOV
6. Less pitch
7. Sharp filter (bone)

Question 17

What is overranging?

Answer 17

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.

Question 18

Beam hardening looks like what? Fixing it?

Answer 18

1. Cupping
2. Dark bands/Streaks

Fixing it:

1. Pre-hardening
2. Calibration Correction
3. Software
4. Avoidance

Question 19

What is the difference between beam hardening and photon starvation and Metal artifacts?

Answer 19

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

Question 20

How do you correct metal artifact?

Answer 20

Increase kVp

Thinner slices

Software

Question 21

Which metals tend to have more artifact?

Answer 21

High Z metals (Iron and platium)

Compared to low Z (titanium)