radiation protection

Question 1

how to express dose

Answer 1

absorbed dose
dose equivalent
effective dose

Question 2

absorbed dose

Answer 2

radiation energy deposited in a unit mass of tissue

Question 3

dose equivalent

Answer 3

absorbed dose weighed for harmfulness of different radiations

Question 4

effective dose

Answer 4

dose equivalent weighted for radiation susceptibility of different tissues
(measure of risk, rather than a physical dose)

Question 5

weighting factors

Answer 5

different parts of the body have different weighting factors

Question 6

how does radiation affect DNA

Answer 6

1) Directly
- secondary electron breaks a DNA strand
2) Indirectly
- the electron leaves a trail of ionised water molecules behind the highly reactive OH free radicals created in this way are very damaging
- more frequent than direct

Question 7

types of effect

Answer 7

1) Deterministic
- direct consequence of a dose above a minimum threshold, increasing in severity with dose
2) Stochastic
- increase likelihood but not severity with increasing dose, with no minimum threshold

Question 8

somatic effects

Answer 8

• those occurring in the inviduial as a result of exposure to a dose of radiation

Question 9

genetic effects

Answer 9

• those which arise in the offspring of an individual exposed to a dose of radiation

Question 10

stochastic effects

Answer 10

Stochastic effects have no threshold but the probability of radiation induced effect increases with dose
Severity is independent of dose
- linear no threshold approach

Question 11

factors to reduce an x ray dose

Answer 11

1) time
2) distance
3) shielding

Question 12

time

Answer 12

• longer you spend near a source of radiation higher the dose
• minimise the exposure time

Question 13

distance

Answer 13

• as you move away from the source the dose rate decreases rapidly according to the inverse square law
• Twise the distance – quarter the dose

Question 14

sheilding

Answer 14

• where shieling is used, it attenuates the radiation reducing the dose
• attenuation is proportional to density

Question 15

chose appropriate radiation factors

Answer 15

1) Filtration
2) kV
- High energy x-ray photons are more penetrating so are needed for thicker patients but will reduce image contrast
3) mA
- Increased current increases the number of photons, increasing the dose and improving image contrast
4) FSD (focus-to-skin distance)
- The closer the focus is to the patient the less the x-rays are scattered so the higher the dose but the image is less noisy
- normally fixed dependant on the collimator
5) Collimation
- Defines beam size. Must be no more than 6cm diameter
- Rectangular collimation will reduce dose