Physics Ch 4 Image Detectors

Question 1

film imaging –> #1 weakness?

Answer 1

fixed narrow dynamic range

Question 2

1 advantage of digital vs film imaging?

Answer 2

wide dynamic range –> ok to under/over-expose a little –> can correct later with wondering –> can use lower dose –> digital system has “higher dose efficiency” than film

Question 3

automatic exposure ctrl (AEC):

• what is its function?
• use of what device?
• exposure time determined by what pt factors? (2)
• controls what factor of XR values?

Answer 3

• timer –> terminate exposure
• ionization chamber –> bw pt & detector
• pt –> density & thickness
• control exposure time (s in mAs) –> QUANTITY of radiation reaching detector

Question 4

what is bit depth?

Answer 4

bits –> # shades of gray

12-bit depth = 2^12 = 4096 shades of gray

Question 5

computed radiography (storage phosphor, cassette-based system) –> amt of light detected –> proportional to what aspect of XR?

Answer 5

incident XR intensity

Question 6

computed radiography (storage phosphor, cassette-based system) –> how “reset” plate? what happens if not reset?

Answer 6

expose to bright white light –> erase image on plate –> can re-use plate

if not reset –> ghosting artifacts

Question 7

digital radiography –> direct –> advantage over indirect? (3)

Answer 7

direct:
- no light –> no lateral dispersion
- fill factor ~100% –> more efficient
- higher detective quantum efficiency (DQE)

Question 8

digital display –> typical standard of care –> #pixels?

Answer 8

3 mega-pixels