Chapter 71 - Computed Tomography

Question 1

1. What is the gantry?

Answer 1

• The opening of the CT bore

Question 2

2. What is cost effective CT imaging?

Answer 2

• Cone beam imaging units

Question 3

3. Cone beam imaging generates (more/less) scatter radiation and (more/less) soft tissue detail compared to traditional CT units

Answer 3

• More
• Less

Question 4

4. What is CT best used for?

Answer 4

• Evaluation of bone and complex anatomy
• Modality of choice for assessing intracranial hemorrhage in acute trauma

Question 5

5. What is spatial resolution?

Answer 5

• The ability to differentiate 2 points that are close together

Question 6

6. What is contrast resolution?

Answer 6

• the ability to differentiate 2 structures that are similar in attenuation

Question 7

7. What is benefit of helical over axial scanning?

Answer 7

• Faster scan times

Question 8

8. What is pitch in helical scanning? What is the formula?

Answer 8

• The movement of the CT table relative to the gantry rotation
• Pitch = table movement in 360 degrees/ collimator width

Question 9

9. Pitch of 1 is the same as ______ acquisition, gantry rotates ___ degrees around slice and no data is lost

Answer 9

• Axial mode
• 360°

Question 10

10. A pitch of less than one implies _____, additional info acquired not likely to improve diagnostic quality of image

Answer 10

• Overscanning

Question 11

11. Pitches greater than one improve _____ of scanning and reduce scan ____

Answer 11

• Efficiency
• Time

Question 12

12. Pitches of up to ___ are used commonly at least ____% of gantry rotates around a slice, and remaining ___% is extrapolated from slice before and after

Answer 12

• 1.5
• 66%
• 34%

Question 13

13. CT images with thicker slices have a (greater/lesser) number of photons detected, increasing the ______ and improving the _____ of the image. Thinner slices improve _____ and reduce _____

Answer 13

• Greater
• Available signal
• Inherent contrast
• Spatial resolution
• Partial volume averaging

Question 14

14. In general, thin CT slices are chosen when ___________ is important and thick slices are chosen when __________ is important

Answer 14

• Special resolution
• Soft tissue contrast

Question 15

15. What are artifacts in CT (4)

Answer 15

• Beam hardening
• Motion artifact
• Partial volume
• Metallic artifact

Question 16

16. What is beam hardening?

Answer 16

• Dark or light bands across the anatomy caused by changes in the intensity of the x-ray beam

Question 17

17. Small movements lead to ____ and large movements lead to ________ because part of patient is scanned in more than one slice

Answer 17

• Blurring
• Double images

Question 18

18. What is partial volume?

Answer 18

• When more than one tissue type is represented in a given voxel

Question 19

19. How can you determine the true anatomy with partial volume artifacts?

Answer 19

• Thinner slices limit partial volume artifacts
• Careful viewing of transverse slices and 2-dimensional reconstructions usually identify the true anatomy

Question 20

20. What factors lead to metallic artifact

Answer 20

• Beam hardening
• Partial volume artifact

Question 21

21. When using contrast media, pathological enhancement of vascular supply, as seen with ___ and ____ will be higher than surrounding tissue

Answer 21

• Neoplasia
• Inflammation

Question 22

22. Some tissues have a higher than normal degree of enhancement with IV contrast. What are they and why?

Answer 22

• Lymph nodes
• Salivary glands
• Due to blood supply or affinity for iodine

Question 23

23. To eval contrast enhancement, a time delay of ____ is recommended before post contrast scan of the distal limbs to facilitate ______ when injecting into jugular vein

Answer 23

• 5 minutes
• Recirculation of the contrast media

Question 24

24. Contrast medium enhanced images of the brain should be obtained __ to __ minutes after injection

Answer 24

• 1.5-3 minutes

Question 25

25. What is CT angiography?

Answer 25

- The use of contrast to highlight vasculature

Question 26

26. If specific catheterization is used, how should contrast be diluted? Why?

Answer 26

- Between 50-100mgI/mL to avoid excessive metallic artifact

Question 27

27. What is dacryocystography? What is it useful for?

Answer 27

- Contrast medium to evaluate the nasolacrimal system

Question 28

28. What is the window level?

Answer 28

- The Hounsfield number around which the other shades of gray are centered

Question 29

29. What is the window width?

Answer 29

- The number of shades of gray that are portrayed on an image

Question 30

30. The human eye can visualize approximately __ to ___ shades of gray

Answer 30

- 30-90

Question 31

31. CT image has how many shades of gray?

Answer 31

- 4096

Question 32

32. To see soft tissues, the number of shades of gray are (increased/decreased) so that subtle differences are more apparent

Answer 32

- Decreased

Question 33

33. When contrast is maximized by decreasing number of shades of gray, what is lost?

Answer 33

- Spatial resolution

Question 34

34. For bone imaging, spatial resolution is maximized by increasing window width to _____ at expense of ____

Answer 34

- About 2000 - Soft tissue contrast

Question 35

1. What is multiplanar reconstruction?

Answer 35

- Reconstruction of a 2D image in any plane

Question 36

2. What is the quality of the reconstruction dependent on?

Answer 36

- Slice thickness used for acquisition - Helical vs axial modes

Question 37

3. T/F: Thicker slices always produce a smoother reconstruction with better spatial resolution in selected planes

Answer 37

- False, thinner slices