Radiation Dose

Question 1

Coherent scattering

Answer 1

• incident photon contacts an outer eletron and becomes a scattered photon
• decreases contrast
• about 8% of interactions in an xray dental beam

Question 2

Photoelectric Absorption

Answer 2

• incident photon contacts and ejects an inner electron and forms an ion pair
• increases contrast
• about 30% of interactions in a dental xray beam

Question 3

Compton scattering

Answer 3

• incident photon contracts an outer electron and forms an ion pair
• decreases contrast
• about 62% of interactions in a dental xray beam

Question 4

dosimetry = exposure

Answer 4

unit of measure = roentgen 1C/kg = 3.88x 10^3R

what it is = energy produced by Xray tube

Question 5

dosimetry = absorbed dose

Answer 5

unit of measure = gray, 1 gray = 100 rads

what it is = energy absorbed in tissue

Question 6

dosimetry = equivelent dose

Answer 6

unit of measure = sievert, 1 sievert = 100 rems

what it is = energy absorbed in tissue multiplied by radiation weighting factor

Question 7

dosimetry = effective dose

Answer 7

unit of measure = sievert, 1 sievert = 100 rems

what it is = energy absorbed in tissue multiplied by tissue multiplied by tissue weighing factor

Question 8

Derministic effects

Answer 8

• hair loss, cataracts, skin damage, oral mucositis
• threshold dose = below this threshold nothing happens
  0. 1 Gy > in-utero birth defects
  0. 5 Gy > cataracts
  3. 0 Gy > radiation burns

Question 9

Stochastic effects

Answer 9

• cancer, lukemia, herritable effects
• linear, no threshold model = there is no threshold dose
• 0.1 Gy > anything below this we can only assume effects are dependent on dose
• 0.0017% risk increase by 1 Gy

Question 10

Radiation chemistry

Answer 10

DIRECT
- direct alteration of bilogical molecues (protein, DNA)
- accounts for about one-third of biological effects
INDIRECT
- ionizing radiation converts water to free radials which alter biological molecules (protein, DNA)
- accounts for about two-thirds of biological effects

• cells that are mitotically active are more radiosensitive
• cells most likely to be damaged first are hematopoietic, epithelial etc.
• cells least likely to be damged are nerve, muscle etc.

Question 11

Dose reduction

Answer 11

• occupational exposure limit is 50mSV per year
• ALARA = as low as reasonably possible
• ALARDA = as low as diagnostically acceptable
• use E/F speed films or digital imaging
• use > 18cm source to object distance to reduce exosure and improve image clarity
• use rectangular collimation