Review Of CT Physics

Question 1

CT scanners need?

Answer 1

• Stable and fast detectors
• Exact source - detector geometrical relationship
• high resolution
• small focal spot
• mono-energetic beam
• large amount of computing power
• strong radiolucent beds

Question 2

CT numbers show?

Answer 2

• attenuation coefficient of materials
• affected by KV
• Air has a CT number of -1000
• water has a CT umber of 0

Question 3

CT numbers

Answer 3

CT numbers = [u(tissue) - u(water) / u(water] x 1000

• Measured in distribution of u
• u values are scaled to that of water to give CT numbers
• multiplayer of 1000 used to make whole numbers
• for a given tissue depends of the KV, scanner model, etc:

+ water = 0
+ Air = -100
+ bone = 1000
Everything else somewhere else in between

Question 4

In filtered back projection?

Answer 4

A filter removes the streaks from the image
The more back projections the better the objects are represented in the image.

Question 5

Image reconstruction

Answer 5

The more projections added - the better the representation

Simple back projection gives a streaky image

A filtered (image projection) is applied - FILTERED BACK-PROJECTION

Variety of filters available - smooth, fine

Question 6

Iterative reconstruction (IR)

Answer 6

Uses acquired information for comparison

Can model physics

Very slow

“Plasticky look”

Question 7

CTDI?

Answer 7

Is dependant on the irradiated slice width and dose profile

Question 8

Dosimetry CTDI

Answer 8

CTDI = Computed tomography Dose index

Defined as CTDI = 1/NT integrated dose profile

NT = number of detector rows width of detector row
Dose profile - penumbra and scatter

Question 9

Weighted CTDI

Answer 9

• doses greater towards the periphery
• measure in outer holes of phantom and add to central ones

CTDI w = 1/3 CTDI central/100 + 2/3 CTDI peripheral/100

• normally displayed on CT console
• indicates dose per rotations

Question 10

DLP?

Answer 10

Is defined as the CTDI vol times the scan length

Can be converted to effective dose

Question 11

DLP and CTDI

Answer 11

CTDI vol = CTDI w / pitch

DLP = dose length product

DLP = length of scan x CTDI vol

• commonly displayed
• indicates affect of settings on scan dose
• can be converted to effective dose if needed

Question 12

Which factors affect dose and image quality?

Answer 12

• Slice thickness
• pitch
• KV/mAs
• contrast
• reconstruction filter

Question 13

Slice thickness (Z-direction) ?

Answer 13

Thinner slices improve resolution in the Z-direction

Wider slices can increase partial volume effects

Question 14

Slice thickness info - narrow slices

Answer 14

More noise

Better Z-axis resolution

Question 15

Slice thickness info - wide slices

Answer 15

Less noise ( more x-ray photons contribute to image)
Worse Z-axis resolution
Partial volume effects? - depends on the size of the object being imaged
Faster coverage
Fewer slices

Question 16

Helical Imaging?

Answer 16

Data is reconstructed using interpolation

Requires overscan so that there is sufficient data for thee beginning and finishing slices

Question 17

Pitch = 1

Answer 17

Similar to axial scan at isocentre
No overlaps or gaps

Question 18

Pitch < 1

Answer 18

Slices overlap
Higher dose
Better image quality
Slower scan

Question 19

Pitch > 1

Answer 19

Gaps in between slices
Lower dose
More interpolation
Faster scan

Question 20

Image reconstruction

Answer 20

• Helix scanning, produces a volume image
• Location of sliced reconstruction is arbitrary
• Data is collected from the projections
• Data is reconstructed into slices via interprolation
  - Data from production angle at different table positions
• Needs 180° of data (+fan) therefore needs to overscan to have this data for the beginning and finishing slice.

Question 21

Kvp ?

Answer 21

Increasing KVP can reduce noise in the image

Can be altered automatically by the scanner

Question 22

Tube potential (kVp)

Answer 22

~ non linear with radiation dose
~ radiation dose proportional to kvp^2
~ higher kvp means less noise in the image
~ however this may reduce low contrast resolution

~ can be manually fixed kvp or automatic selection
~ typically 120Kvp (between 80 - 140 kvp)
~ lower kvp for paeds and contrast protocols

Question 23

MA ?

Answer 23

• Noise decrease as mA increases for a fixed rotation time
• dose increases linearly with mA
• is adjustable

Question 24

MA is the

Answer 24

Tube current

Question 25

Which of the following are true?

1. Partial volume is where the values of a voxel are averaged
2. Beam hardening causes areas deeper in the artefact to appear more attenuating
3. Ring artefacts are caused by detector non-uniformities
4. Photon starvation is caused by areas of high attenuation
5. Metal can cause beam hardening, scanner, noise and photon starvation
6. Ring artefacts are caused by jewellery
7. Out of field artefacts are caused by anatomy outside the reconstructed FOV

Answer 25

1. 3. 4. 5.7.

Question 26

Beam hardening

Answer 26

Cupping - areas deeper within objects appear less attenuating

If medium is known correction can be applied

Question 27

Photon starvation

Answer 27

• occurs in large patients or thick regions
• high attenuation gives parts of projections with low photon numbers
• noise is magnified during reconstruction, giving streak artefact.

Question 28

Ring artefacts

Answer 28

Non-uniformities amongst detectors
Appears a a ring

Question 29

Out of field artefacts

Answer 29

Cause of streaks and errors in reconstructed FOV due to presence of photon, interacting, medium outside FOV.

Material not accounted for in reconstruction

To mitigate this, try wide bore or arms up.