33. CT: COLLIMATORS

Question 1

1. What are Collimators?

Answer 1

• they are used in X-Ray imaging
• they are found at the exit of the X-Ray Tube
• they are used to find the size and the shape of the
  X-Ray beam

Question 2

2. What do CT units make use of?

Answer 2

• they make use of Collimators

Question 3

3. What does the interaction of X-Rays with tissue create?

Answer 3

• it creates randomly scattered photons
• these create image noise
• this clouds the image
• this decreases the image quality

Question 4

4. What role do Collimators play when it comes to X-Rays?

Answer 4

• they are found at the front of the detector
• they act as an anti-scatter grids

Question 5

5. What do these anti-scatter grids do?

Answer 5

• they eliminate X-Ray photons
• these deviate from a straight source-detector path
• they reduce the beam size
• they limit the beam thickness

Question 6

6. What does the presence of Collimator on the front of the detector result in?

Answer 6

• it results in a small apparent focal spot

Question 7

7. What are Artefacts?

Answer 7

THEY ARE THE DISCREPANCIES THAT ARE FOUND BETWEEN:
- the CT numbers represented in the image
- AND the expected CT numbers (in HU units)

Question 8

8. What are 5 types of Common Artefacts?

Answer 8

1. Beam Hardening
2. Partial Volume Effect
3. Bad Detectors
4. Metal
5. Patient Motion

Question 9

9. What 2 types of Collimators do we have when it comes to CT Scanners?

Answer 9

1. THE ONE THAT IS PLACED AT THE FRONT OF THE
   X-RAY TUBE:
   
   • this is known as the Pre-Collimator
   • this reduces the beam size
   • this limits the beam thickness
   • this results in a smaller focal spot
   • results in better image quality
2. THE ONE THAT IS IN CONTACT WITH THE DETECTOR:
   
   • this is known as the Post Collimator
   • this one acts as an Anti- Scatter Grid
   • it absorbs the Scatter Radiation
   • results in better image quality

Question 10

10. What is the Beam Hardening Effect?

Answer 10

• this is when the X-Ray beam is hardened by passing
  through an object
• this happens specifically in denser objects
• specific areas of the image will then appear darker
  than they should be

Question 11

11. What happens in extreme Beam Hardening cases?

Answer 11

• a detector element may fail
• this results in a constant output signal

Question 12

12. What is a primary source of image noise?

Answer 12

• detectors
• their associated signal amplifiers

Question 13

13. What reduces the amount of image noise?

Answer 13

• Electronic Noise Suppression Filters
• Software Noise Suppression

Question 14

14. Who can reduce the noise in an image?

Answer 14

• the operator
• provided that this loss of detail is acceptable

Question 15

15. What kind of beams can X-Rays not produce?

Answer 15

• Monochromatic beams
• this is why an artefact related to beam hardening is
  common

Question 16

16. What does Beam Hardening result in?

Answer 16

• the image can have streaks and shading
• the image can be interpreted inaccurately

THE STREAKS:
- will be specifically seen in regions that are very dense
- such as the bone

Question 17

17. When are lower energy X-Rays absorbed?

Answer 17

• when the energy peak will shift towards higher
  energies
• the absorption values will be underestimated

Question 18

18. How can we reduce Beam Hardening?

Answer 18

• by reducing the pre-hardening of the beam
• this is done by placing a thin metal plate in front of the
  tube

Question 19

19. What materials can the thin metal plate made out of?

Answer 19

• Molybdenum
• Tungsten

Question 20

20. What can compensate for the Beam hardening in the projections?

Answer 20

• applying appropriate algorithms in the reconstruction

Question 21

21. What does this image show?

Answer 21

• The Beam Hardening Effect

Question 22

22. When does the Partial Volume Effect occur?

Answer 22

• it occurs whenever a pixel represents more than one
  kind of tissue
• it can also occur when a single voxel contains a variety
  of tissue
• this occurs in CT and Nuclear Imaging

Question 23

23. When is the Partial Volume Effect particularly relevant?

Answer 23

• when a tissue boundary lies within a CT splice
• this means that one splice contains information from
  the previous splice

Question 24

24. What does the Partial Volume Effect result in?

Answer 24

• it blurs the intensity distinction
• this happens between the adjacent tissues

Question 25

25. What can reduce the Partial Volume Effect?

Answer 25

- higher resolutions - repositioning of the patient - we can use thinner slices

Question 26

26. What do these images show?

Answer 26

- the Partial Volume Effect