CT

Question 1

role of filter

Answer 1

to account for the attenuation through the patients body

Removes low energy x-rays
Produces a more monochromatic beam
May be bowtie-shaped to even out attenuation once it passes through the body

Question 2

in CT : where is a collimator placed

Answer 2

between filter and the patient.

Question 3

Aim of collimator in CT

Answer 3

lower dose to patient

restrict scatter outside desired slice

Question 4

what are the two types of detector in CT

Answer 4

SSD and ionisation chamber (no longer used)

Question 5

Solid state detectors in CT operate how.

Answer 5

There is a solid scintillator layer that converts the x-rays into visible light photons.
The photodiode then converts the photon input into an electrical signal.

Question 6

what type of CT scanner is most used?

Answer 6

3rd generation. wide row of detectors rotates with the xr beam.
takes 0.3s to image a slice as the whole beam is detected

Question 7

advantages of spiral scanning in CT

Answer 7

no breathing artefact
better use of contrast
overlapping slices better reconstruction
high pitch can reduce scan rime / dose

Question 8

what is Pitch

Answer 8

distance couch travels in a single rotation / width of slice

Question 9

what is the difference betweena pitch of 0.5, 1 and 2

Answer 9

0.5 they are overlapping. travel 3cm / beam width 6cm
1 - equal
2 - bits are missed

Question 10

Advantage of high pitch number

Answer 10

Lower radiation dose

quick scan

Question 11

Disadvantage of low pitch number

Answer 11

more sparsely sampled

Question 12

what do multislice scanners allow you to do?

Answer 12

Allows to choose the slice thickness (as well as the collimator doing this)

Question 13

benefit of Multislice scanners?

Answer 13

Faster scanning (better dynamic imaging)
thinner slices
3D imaging
muliple slices all at the same time can be acquired.

Question 14

What are the types of detector arrays?

Answer 14

Linear
Adaptive
Hybrid

Question 15

Adapative array has what configuration of elements

Answer 15

The elements within the central detector rows are the thinnest and they get wider towards the outside.

Question 16

What is the benefit of adaptive array

Answer 16

fewer detector rows so fewer septae dividing rows. This improves dose efficiency.

Question 17

multislice pitch - two ways to measure it are what

Answer 17

Pitch d

Pitch x

Question 18

pitch D =

Answer 18

couch travel per rotation / width of xr beam

doesnt’ represent the overlapping xr beam

Question 19

pitch x =

Answer 19

couch travel per rotation / total width of simultaneously acquired slices.

Question 20

How is slice thickness determined differently in Single slice CT and Multislice CT

Answer 20

Single slice CT = determined by collimation. Limited by detector row width.
Multisclice CT = determined by width of detector rows

Question 21

HU are a representation of what

Answer 21

LAC

Question 22

HU =

Answer 22

HU = 1000 x (μt – μw) / μw

uw is water.

Question 23

What are the issues with back projection?

Answer 23

1. Too few projections cause artefacts in the image as there are too few directions of summed LACs to accurately represent the image. Typically 2000 projections are used.
2. averging of values means edges can be blurred.

Question 24

How is the averaging of blurred lines in back projection correct?

Answer 24

Filtered backprojection

Question 25

Iterative reconstruction

Answer 25

basically compares an initial image with what the detectors should have received for the image/patient. it continually updates the image.

Question 26

iterative reconstruction benefit

Answer 26

reduces doses but is a lengthy calculation.

Mostly used

Question 27

What is dual energy CT

Answer 27

Dual-energy CT (DECT) utilises the photoelectric effect to separate out different materials within the voxel based upon their different attenuations at different beam energies.

Question 28

What are the different modalities for dual energy CT?

Answer 28

Dual source DECT

Singe source DECT

-Fast kVP switch. -

Question 29

-Fast kVP switch. -

Answer 29

flips.
- detector based spectral CT.
- the detector has two layers than can detect two energies.

Question 30

Singe source DECT

,

Answer 30

• two detectors. can be consecutive scans of different energies, sequential each rotation is repeated twice

Question 31

Dual source DECT

Answer 31

• two tube, differnet voltages, 90 degress to each other. (increased doseand scatter)