Risk and Dose Flashcards
what is meant by absorbed dose (D) and unit
measure of amount of energy absorbed from radiation beam per unit mass of tissue, Gray (Gy, 1 Joule/kg)
explain how absorbed dose can vary in effect even if it is the same amount
depends how the energy is packaged: more damaging if energy fired in one go
explain equivalent dose (H) and units
allows potential harm from different types of radiation to be considered
H= absorbed dose (D) x radiation weighting factor (Wr)
unit= sievert (Sv)
radiation weighting factor (Wr) for
a) X-rays and gamma rays
b) beta particles
c) protons and neutrons
d) alpha particles
radiation weighting factor (Wr) for
a) X-rays and gamma rays: 1
b) beta particles: 1
c) protons and neutrons: 5-10
d) alpha particles: 20
interpret the radiation weighting factor of x rays and gamma rays
equivalent dose in sieverts= absorbed dose in Grays
explain effective dose (E) and unit
allows doses from different investigations or different parts of body to be compared, because some parts of body more radiosensitive than others
effective dose= dose equivalent x tissue weighting factor (Wt) (sieverts)
if more than 1 organ exposed, effective dose is sum of all effective doses to all exposed organs
what is the most radiosensitive part of the body?
gonads
what do each of these dosimetry abbreviations stand for:
D
H
E
D: absorbed dose
H: equivalent dose
E: effective dose
what medical examination gives the highest radiation dose?
CT chest
what medical examination gives the lowest radiation dose?
dental intraoral film
how long would it take to receive equivalant dose from background radiation compared to effective dose of intraoral x rays?
16 hours
how long would it take to receive equivalant dose from background radiation compared to effective dose of dental panoramic film?
3 days
how many mSv from
a) flight to Spain return
b) UK average
c) Cornwall
d) intraoral x rays
e) oral panoramic
f) chest x ray
g) head CT
h) barium meal
i) chest CT
j) mammography
a) flight to Spain return: 0.02mSv
b) UK average: 2.6 mSv/yr
c) Cornwall: 7.8mSv/yr
d) intraoral x rays: 0.001-0.008mSv
e) oral panoramic: 0.02mSv
f) chest x ray: 0.02mSv
g) head CT:2mSv
h) barium meal:3mSv
i) chest CT:8mSv
j) mammography:1.24mSv
risk of malignancy from 1 intraoral film using
a) traditional x-ray set and D-speed film
b) 70kV x-ray set and E-speed film
a) traditional x-ray set and D-speed film: 1/ 2 million
b) 70kV x-ray set and E-speed film: 1/20 million
risk of malignancy from 1 panoramic film
multiplication risk factors for age groups
0.21-1.9/ 1 million
explain collective dose
total effective dose to population
collective dose = effective dose x population
unit= man-Sv
3 groups of population for purposes of limiting radiation dose
patients
occupationally exposed (workers)
general public
4 things to consider before prescribing pt an x ray
- correct assessment of indications
- expected diagnostic yield (how useful is x ray)
- way in which results influence diagnosis/ treatment
- clinician having adequate knowledge of physical properties and biological effects of ionising radiation
2 classifications of radiation workers
classified (>6mSv / year)
non-classified
what are comforters and carers and dose limits
people who willingly undergo radiation exposure to support/ comfort pt, eg parent or carer
no dose limits but dose constraints
3 golden rules of dose reduction
distance
shielding
time
distances of controlled area
1m for sets up to 70kV
1.5m for sets above 70kV
(preferably operator >2m away from pt and x ray tube, not in line of primary beam)
what law of physics does radiation follow? explain
inverse square law
intensity of radiation reduces to 1/4 as distance is doubled
other rules of controlled area
- warning light illuminated when x ray on
- warning notice provided when controlled area extends to door
4 materials used for shielding
- lead
- lead rubber/glass
- concrete
- barium plaster
2 ways to reduce dose to operator and comforter/ carer
- stand behind lead glass screen
2. lead-rubber apron
when to use thyroid collars
if thyroid gland in primary beam
what is collimation?
x ray beam shaping to minimum required to cover image receptor
6 ways to use dental x ray set to reduce x ray dose
- higher kV (60-70kVp)
- rectangular collimation
- adequate aluminium filtration
- long focus to skin distance (>20cm)
- accurate timer
- adequate warning signals
how to reduce dose by quality assurance
- regular checks of equipment (x-ray and processing)
- image quality evaluation and audit
advantage of digital imaging over film 5
- quicker
- less pt dose
- less storage requirements
- constantly evolving
- environmentally better
2 dose measurements in radiography
- entrance surface dose (ESD) miliGrays, mGy. max dose to tissues, measured using thermoluminescent dosimeters (TLDs), ionisation chambers
- dose-area product (DAP): max dose to tissues
who should have personal dosemeters?
people exposed to more than 1mSv/year
>100 intraoral or 60 panoramic films per week
how do film badges work
contain photographic film to show how much radiation youve been exposed to
advantages and disadvantages of film badges
advantages:
- cheap
- permanent record
- may be reassessed
- can see type of radiation received
disadvantages:
-processing required
how do thermoluminescent dosimeters (TLD) work?
lithium fluoride crystals release stored radiation energy as light when heated
advantages and disadvantages of TLDs
advantages:
- sensitive
- reusable
- provide quick read-out
- no processing required
disadvantage:
- destructive read-out
- expensive
- limited info on energy of radiation received
