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RD1034 Scientific Principles of Medical Imaging > dosimetry > Flashcards

dosimetry Flashcards

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USMLE® Step 1 flashcards
1
Q

what is dosimetry?

A

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

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2
Q

what is the importance of dosimetry

A
  • monitoring of exposures
  • identification of variation
  • legislation
  • protection of patients and staff
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3
Q

what are the quantities and units of exposure

A
  • exposure (X)
  • Kerma (K)
  • Absorbed dose (D)
  • Equivalent dose ( Ht)
  • effective dose (E)
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4
Q

what is required to measure radiation dose

A

a dose-meter

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5
Q

what does a dose-meter contain

A

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

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6
Q

what are the two types of dose-meters

A
  • absolute
  • practical
  • thermoluminescent dosemeter (TLD)
  • film
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7
Q

absolute dose meter

A
  • very specialised equipment
  • kept as national physical laboratory
  • used under carefully controlled conditions
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8
Q

practical dose meter

A
  • smaller and more compact
  • used in hospitals
  • calibrated regularly against absolute dose meter
  • sensitive to external factors
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9
Q

what is intensity

A

measure of number of photons in the beam

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10
Q

what is the ionising effect on air

A

exposure (X)

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11
Q

what is an air ionisation chamber

A

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

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12
Q

what are some examples of applications of air ionisation chambers

A
  • 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
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13
Q

what is air kerma

A

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

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14
Q

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

A

34eV
- very sensitive process

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15
Q

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

A
  • exposure (X)
  • dose in air (Dair)
  • air kerma (Kair)
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16
Q

what is the calibration factor

A

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

17
Q

what type of chamber is practical dosimeter

A
  • thimble ionisation chamber
18
Q

thimble ionisation chamber

A

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

19
Q

what happens to thimble ionisation chamber when irradiated

A

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

20
Q

why ionisation in air?

A
  • readily available
  • composition close to being universally constant
21
Q

what is dose a measure of

A

average energy absorbed from radiation beam into medium

22
Q

what does air ionisation chamber measure

A
  • exposure (X)
  • measure absorbed dose in air
  • air kerma
23
Q

absorbed dose

A

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

24
Q

equivalent dose

A

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

25
Q

effective dose

A

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

26
Q

what is the dose area product meter

A

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

27
Q

what does the DAP measure

A

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

28
Q

radiographic film

A

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

29
Q

TLDs

A

thermoluminescent dosimeter

30
Q

what are TLDs used for

A

used for measuring x-rays and beta radiation exposures

31
Q

what are features of TLDs

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

RD1034 Scientific Principles of Medical Imaging (7 decks)

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