34. CT: RING ARTEFACTS

Question 1

1. What causes Ring Artefacts?

Answer 1

• there is a problem with the detector
• uncalibrated detectors create ring artefacts in the
  reconstruction

Question 2

2. What are the detectors a critical element of?

Answer 2

• they are a critical element of the unit

Question 3

3. How are detectors centred?

Answer 3

• they are centred with respect to the source
• otherwise the image would be blurred
• each detector uses a separate ring of an atom

Question 4

4. What has to be done to detectors?

Answer 4

• they have to be calibrated

Question 5

5. What does calibration ensure?

Answer 5

IT ENSURES THAT:
- the output signal intensity is identical for all the
detectors that have the same incident X-Ray intensity

Question 6

6. What does this image show?

Answer 6

• the Ring Artefacts

Question 7

7. What do Metal Artefacts result in?

Answer 7

• they cause Streaking Artefacts
• this is due to parts of the Projection Data being
  blocked
• it is s result of the presence of metallic objects during
  surgeries
• the metal is too dense and wide
• it will absorb more of the X-Rays
• it will appear as extremely white in the image

Question 8

8. What are some examples of Metal Artefacts?

Answer 8

• dental fillings
• prosthetic devices
• surgical clips

Question 9

9. How do we reduce Metal Artefacts?

Answer 9

• remove the metal material as much as possible

Question 10

10. What does this image show?

Answer 10

• Metal Artefacts that cause Streaking Artefacts

Question 11

11. What is Patient Motion?

Answer 11

• this is the motion blur that occurs when a patient
  moves
• this happens even if the patient breathes
• it happens during the scan of one slice
• it blurs the scan

Question 12

12. Can Patient Motion blur be corrected?

Answer 12

• it cannot be corrected

Question 13

13. How can we reduce the risk of Motion Blur?

Answer 13

• shorter acquisition times
• asking the patient to hold their breath
• using Helical Scanning Principles

Question 14

14. Images of which organ continue to pose problems?

Answer 14

• detailed images of the heart

Question 15

15. What does this image show?

Answer 15

• Metal Artefacts that result in Streaking Artefacts

Question 16

16. What is Photon Starvation?

Answer 16

• we use a lower mass
• there are not enough photons that reach the detector

Question 17

17. What does this image show?

Answer 17

• Metal Artefacts that result in Streaking Artefacts

Question 18

18. What is the result of having less projections?

Answer 18

• we obtain images from fewer angles
• we decrease the image resolution

Question 19

19. What is the result of reducing the X-Rays?

Answer 19

• the quality of the image will decrease
• the patient radiation dose increases