dosimetry

Question 1

what is dosimetry?

Answer 1

the measurement of a dose caused by radiation deposited in a medium
aka measurement of radiation from x-ray exposure

Question 2

what is the importance of dosimetry

Answer 2

• monitoring of exposures
• identification of variation
• legislation
• protection of patients and staff

Question 3

what are the quantities and units of exposure

Answer 3

• exposure (X)
• Kerma (K)
• Absorbed dose (D)
• Equivalent dose ( Ht)
• effective dose (E)

Question 4

what is required to measure radiation dose

Answer 4

a dose-meter

Question 5

what does a dose-meter contain

Answer 5

some sort of material whose response to radiation is measurable
produces an electrical signal when irradiated

Question 6

what are the two types of dose-meters

Answer 6

• absolute
• practical
• thermoluminescent dosemeter (TLD)
• film

Question 7

absolute dose meter

Answer 7

• very specialised equipment
• kept as national physical laboratory
• used under carefully controlled conditions

Question 8

practical dose meter

Answer 8

• smaller and more compact
• used in hospitals
• calibrated regularly against absolute dose meter
• sensitive to external factors

Question 9

what is intensity

Answer 9

measure of number of photons in the beam

Question 10

what is the ionising effect on air

Answer 10

exposure (X)

Question 11

what is an air ionisation chamber

Answer 11

radiation detector used for determining intensity of a beam of radiation or for counting individual charged particles

Question 12

what are some examples of applications of air ionisation chambers

Answer 12

• as detectors in automatic exposure devices (AEDs)
• measure exposure (X) accurately
• measure x-ray tube output and perform QA tests on x-ray equipment
  -a dose area product (DAP) meters

Question 13

what is air kerma

Answer 13

the number of individual x-ray photons per unit of area
- doesnt say anything about the extent of the x-ray beam

Question 14

what is the energy to create one ion pair in the air

Answer 14

34eV
- very sensitive process

Question 15

what can a calibrated air ionisation chamber provide an accurate measurement of

Answer 15

• exposure (X)
• dose in air (Dair)
• air kerma (Kair)

Question 16

what is the calibration factor

Answer 16

ratio of absolute dose meter measurement of exposure to practical chambers reading delivered under standard conditions

Question 17

what type of chamber is practical dosimeter

Answer 17

• thimble ionisation chamber

Question 18

thimble ionisation chamber

Answer 18

every measurement made by practical ionisation chamber must be multiplied by calibration factor

Question 19

what happens to thimble ionisation chamber when irradiated

Answer 19

electrons liberated in wall and attracted to electrode
central electrode will lose some of its positive charge
electrometer measures charge collected

Question 20

why ionisation in air?

Answer 20

• readily available
• composition close to being universally constant

Question 21

what is dose a measure of

Answer 21

average energy absorbed from radiation beam into medium

Question 22

what does air ionisation chamber measure

Answer 22

• exposure (X)
• measure absorbed dose in air
• air kerma

Question 23

absorbed dose

Answer 23

biological effects in any tissue are proportional to amount of energy absorbed per unit mass of tissue

Question 24

equivalent dose

Answer 24

we need to consider how absorbed dose is influenced by biological effects of different radiations

Question 25

effective dose

Answer 25

different organs of body are assigned tissue weighting factors (Wt) to reflect different sensitivity of each organ to radiation damage

Question 26

what is the dose area product meter

Answer 26

parallel plate ionisation chamber connected to electrometer where the radiation beam never exceeds area of chamber

Question 27

what does the DAP measure

Answer 27

entire area of x ray beam exposure
measures radiation dose to air multiplied by area of x-ray field

Question 28

radiographic film

Answer 28

an emulsion containing radiation sensitive silver halide crystals and a transparent, flexible blue tinted base make up x-ray films for general radiography

Question 29

TLDs

Answer 29

thermoluminescent dosimeter

Question 30

what are TLDs used for

Answer 30

used for measuring x-rays and beta radiation exposures

Question 31

what are features of TLDs

Answer 31

• versatile material
• different forms; powder, chips, discs or rods
• small size - a few ml
• reusable
• reliable if calibrated