Chapter 27 - CT

Question 1

Origin of word tomography

Answer 1

Tomos = a section/cutting (greek)
Graphein = to write (greek)

Question 2

CT numbers for various things

Answer 2

air -1000 HU
bone > 1000 HU
fat -20 to -100 HU
water 0 HU
Muscle/blood 40-60 HU

Question 3

Spiral CT vs single slice sequential CT

Answer 3

Single slice can produce similar result but

1) increase time
2) increase radiation dose
3) less artifact

Question 4

Split bolus technique

Answer 4

alternative ways of administering IV contrast

split the contrast bolus to arterial and venous and therefore only need to scan once

Question 5

Tube voltage measurement

Answer 5

kVp
value that determines energy level of x-ray tube
higher voltage = better tissue penetration; decrease relative contrast difference

higher voltage needed in fat people

Question 6

Current time product key points

Answer 6

mAs

higher mAs represent more radiation but decrease noise

Question 7

Collimation define

Answer 7

method to reduce thickness of x-ray beam

Question 8

Pitch define

Answer 8

ratio of table feed and collimation

P = table feed / single-section collimation

Question 9

Define table feed

Answer 9

speed at which table moves during tube rotation

Question 10

Dual energy CT benefit

Answer 10

Simultaneous high and low kVp

1) reduces volume/rate of contrast administration
2) low kVP gives better contrast of image
3) high kVp gives less noise

Question 11

Post-scan parameters of CT

Answer 11

1) increment: spacing of reconstructed image from raw set (1-5 mm)
2) slide width: thickness of each slice (0.5-10 mm)
3) field of view: lower FOV better resolution
4) Windowing: Window width sets number of gray scales; window level defines middle gray-scale value of width
5) reconstruction algorithm: filtered back projection and iterative reconstruction

Question 12

Dynamic CT scanning key points

Answer 12

Without moving table, imaging at fixed position with continuous rotation of CT gantry

Question 13

Post-processing

Answer 13

1) multiplanar reformating
2) measurements/centerline
3) 3D reconstruction with shaded surface display
4) maximum intensity projection

Question 14

Difference between shaded surface display and maximum intensity projection

Answer 14

SSD = 3D recon to show depth
MIP = 2D projection to better show calcific structures

Question 15

Partial-volume effect

Answer 15

Objects only partly included that appears like a non-existent lesion

Question 16

Beam-hardening artifacts

Answer 16

Interfaces between material with large difference in density causing streak/scatter

Question 17

Motion artifact treatment

Answer 17

slow breath

generally do not ask dyspneic patients to hold breath because it will result in large artifacts

Question 18

Averaging artifact

Answer 18

loss of information on small lesions due to averaging of the nearby slices

Question 19

Stair-step artifact

Answer 19

recon on spiral CT too large and hence appears to look like a stepped appearance