CT worksheet

Question 1

1. How does CT overcome the limitations of conventional X-ray studies?

Answer 1

There are minimal superimpositional structures as the image can be reconstructed later.

Question 2

2. Describe the two methods of data acquisition (collection) in CT? Which is more common?

Answer 2

The conventional slice by slice and the volume data acquisition.

Question 3

3. What happens as the X-ray beam passes through the body

Answer 3

Some x-rays will be transmitted and contribute to the image as useful signals, some will be absorbed which will also be useful as useful signals by being partly absorbed and appearing brighter and some will be scattered.

Question 4

5. What is the Hounsfield scale?

Answer 4

The Hounsfield scale is a scale used to determine what type of body part is shown based on its shade of grey which is determined by its density. It is derived from the relative sizes of x ray attenuation of the tissue compared to water.

Question 5

6. What are the differences between window width and window level?

Answer 5

Window width controls the contrast while the window level controls the density.

Question 6

7. Which tissue is best imaged as W:1500 L:-600

Answer 6

Lung tissue

Question 7

8. Can you list the key components of the CT system and state what each element does?

Answer 7

X-ray tube – Releases the x-ray photons,
gantry – holds the x-ray tube in addition to the detector array and the high voltage generator,
filters – determine which range of x-ray photon energies can pass through,
collimators – opens or closes to control scatter radiation,
detectors – arranged in an arc to detect x-rays that passed through the body,
moving couch – holds the patient in a supine position.

Question 8

9. How did slip ring technology revolutionise CT imaging?

Answer 8

Slip rings allowed electricity to be conducted by brushes that touch the rings which allowed continuous detector array rotation and introduced volume scanning.

Question 9

10. What advantages does MDCT have over SSCT?

Answer 9

MDCT(Multi detector CT) allows simultaneous acquisition of thick and thin slices over large volumes. Thick slices produce low image noise while thin slices allow high quality multi-planar reconstructions (MPRs)

Question 10

11. What features do CT X-ray tubes have then enable them to dissipate heat quickly?

Answer 10

Large anode discs, high speed rotors, direct oil cooling of the anode, and advanced manufacturer design to use focal spot cooling algorithms

Question 11

12. What are the ideal properties of a CT detector?

Answer 11

High dynamic range, high efficiency, robust and stable, fast response and little after glow

Question 12

13. What is the purpose of the collimators in CT?

Answer 12

Collimators reduce the patient dose and image quality

Question 13

15. Complete the following table by stating increase or decrease or no change assuming no other changes made.
    Alteration
    -Increase kVP
    -Decrease mAs
    -Decrease slice thickness
    -Decrease FOV
    -Increase the matrix from 512x 512 to 1024 x 1024
    -Use of iterative reconstruction rather than filtered back projection
    -Increasing pitch

Answer 13

Alteration
Increase KVP
- effect on spatial resolution decreases 
- effect on SNR increases 
- effect on  radiation dose increases 

Decrease mAs

• effect on spatial resolution increases
• effect on SNR decreases
• effect on radiation dose decreases

Decrease slice thickness

• effect on spatial resolution decrease
• effect on SNR decrease
• effect on radiation dose increase

Decrease FOV

• effect on spatial resolution increase
• effect on SNR decrease
• effect on radiation dose no affect

Increase the matrix from 512x512 to 1024x1024

• effect on spatial resolution increase
• effect on SNR decrease
• effect on radiation dose no affects

use of iterative reconstruction rather than filtered back projection

• effect on spatial resolution increase
• effect on SNR increase
• effect on radiation dose decrease

Increasing pitch

• effect on spatial resolution decrease
• effect on SNR decrease
• effect on radiation dose decrease

Question 14

type of artifact is demonstrated in the images A-C and suggest a remedial solution

1. Ring artefact:
2. Motion artefact:
3. Streak artefact

Answer 14

1. Calibrate the detector
2. Make sure the patient can stay still, faster scan, ECG gating
3. Use newer reconstruction techniques or metal artefact reduction software.