Radiation Measurement & Dosimetry

Question 1

Give 5 reasons why we have to measure radiation doses

Answer 1

record dose given to RT patient, record dose given in a DR imaging procedure, record dose staff receive (medical and non-medical), measure radiation received by public, estimate harm to above categories when used in conjunction with risk factors

Question 2

Define absorbed dose (D) - equation and unit, and a however

Answer 2

this is the energy (E) absorbed per unit mass (m) of the absorbing material
D = E/m - joules per Kg
Units = gray (Gy)
The absorbed dose 1 gray if joule of energy is absorbed her 1kg of the object being irradiated
However, equal absorbed doses may not necessarily produce equal biological effects

Question 3

What 3 factors does the risk of harm to tissue from an exposure to ionising radiation depend on

Answer 3

Absorbed dose (D)
type of radiation absorbed (e.g. X/Gamma Rays)
body organs or tissue that is exposed to radiation

Question 4

Describe the effect of using different types of radiation

Answer 4

they won’t produce the same biological impact, even when the dose or energy delivered to the tissue is the same

Question 5

Describe equivalent dose - units and equation

Answer 5

this is the quantity that expresses the relative biological impact of the radiation and uses radiation weighting factors (WR) to achieve this
measures in sieverts (Sv)
Equivalent Dose (Sv) = Dose (Gy) x WR

Question 6

Describe Weighting Factors (WR)

Answer 6

the value of the radiation weighting factors is a characteristic of each specific type of radiation, radiation in use for medical imaging all have WR values one, therefore where Sv = Gy x WR , Sv is numerically equal to Gy (e.g. this means 1 mSv from gamma radiation = same biologically impact as 1 mSv from X radiation

Question 7

Define Dose Rate

Answer 7

this is the rate of exposure to ionising radiation, (e.g. dose of 100 mSv in 1 day is more damaging than the same dose received over a month), Dose Rate = Dose/Time, can be grays/sieverts/millisieverts/minute/hour

Question 8

Define Effective Dose

Answer 8

this is the quantity which takes into account specific body tissue (because different tissues/organs have different radiosensitivity) that have been irradiated to arrive at a single figure of whole body dose, to assess the risk to a patient

Question 9

Describe Different Tissue Weighting Factors (WT)

Answer 9

different tissues are assigned different WT’s in order to be able to compare risk between 1 procedure and another and the risk of developing a fatal cancer, it’s related tp radiosensitivity of a particular organ/tissue and therefore their susceptibility to damage, gives indication of effect of irradiating all the tissues - an effective whole body dose

Question 10

Give the formula for Effective Dose

Answer 10

Effective Dose (Sv) = Sum of Equivalent Doses (Sv) x WT

Question 11

Describe Dose-Area Product (DAP)

Answer 11

this quantity is commonly displayed on the X-Ray console, varies with exposure factors and the field size used, commonly shown in units of cGy cm squared

Question 12

What does the DAP represent

Answer 12

way of comparing doses for the same exam across different departments and can identify good/poor practice, may be used with conversion factors to provide an estimation of effective dose

Question 13

Summarise Absorbed Dose

Answer 13

takes into account how much energy is absorbed per unit mass of the absorber (J.Kg to the -1)

Question 14

Summarise Equivalent Dose

Answer 14

takes into account the type of radiation, using radiation weighting factors (WR) - (Sv)

Question 15

Summarise Effective Dose

Answer 15

takes into account the radiosensitivity of irradiated tissue, using tissue weighting factors (WT) - (Sv)

Question 16

Summarise Dose-Area Product

Answer 16

is a quantity easily incorporated into diagnostic exam, displayed after exposure using absorbed dose and size of area irradiated (Gy.cm squared)