Measurement of radiation dose Flashcards
Define absorbed dose
energy imparted to organ or tissue per unit mass
- average absorbed dose to that tissue
What is absorbed dose measured in?
gray (Gy)
Define equivalent dose
absorbed dose to organ or tissue weighted for type of radiation
What is equivalent dose measured in
Sievert Sv
Define effective dose
absorbed dose to organ or tissue weighted for type of radiation AND type of tissue
what is effective dose measured in
sieverts
Effective dose is directly related to risk of average person irradiated, what 3 things does this risk depend on
- amount of energy absorbed per uni mass
- type of radiation
- radio sensitivity of site where radiation absorbed
Why might radiation have increased risk in children than adults
- they have longer for cancers to be expressed
- developing tissues may be more radiosensitive
What are 3 ways of dose calculation methods in patients in DR?
- ESD measurement
- DAP measurement
- DLP measurement
What does ESD, DAP and DLP stand for ?
entrance surface dose
dose area product
dose length product
What is entrance surface dose and how does it work + units
- measurement of absorbed dose (mGy)
- directly measured at patients skin using TLD
- calculated from known tube output and other known factors
What does FIA and BSF stand for
fraction incident beam absorbed
backscatter factor
what is dose area product and how does it work + units
- measures product of absorbed dose (in air) and x-ray field area
- mGy.cm^-2
- independent of focus to skin distance (FSD)
- measured directly using a dose area product metre (flat area parallel place IC mounted to the tube port)
What is dose length product and how does it work + units
- mGy.cm
- Directly calculated from CTDI and scan length
What does CTDI stand for
computed tomography dose index
What are Monte Carlo simulations?
- mathematical modelling of a limited range of photons
- frequently used in nuclear medicine dosimetry taking into account half life, radionuclide emission data and biological clearance
- also used in radiotherapy for beam modelling
What does DRL / NDRL / LDRL stand for?
diagnostic reference level
national diagnostic reference level
local diagnostic reference level
what is diagnostic references level
(in IRMER)
- dose levels in medical radiodiagnostic practices or in case of radioactive medicinal products, levels of activity for typical examinations for groups of standard-sized patients
What are the difference ways you would calculate dose in plain film, ct, nuclear medicine and routine fluroscopy?
Plain film = DAP
CT = DLP
Nuclear medicine = activity
Routine fluoroscopy = DAP
what are the disadvantages of using N/LDRL?
- effective dose has limitations
- weighting factors are tailored to a population
- not ideal for individual patient risk
- useful for dose comparisons across modalities
What does ICRP stand for?
international commission on radiological protection
what are the 3 principles from ICRP that controls dose reduction?
- ALARP
- limitation (time, shielding, distance)
- justification (benefit vs risk)
- optimisation (minimum dose for diagnostic image)
Compare the risk of radiation to stages of a growing foetus
radiation risks are most significant during organogenesis (formation fo organs of organism from embryonic cells) and in early foetal period
What is the threshold exposure that typically induces issues with the dental nervous system in foetal?
100 mGy
(worth 3 pelvic CT examinations or 20 x-rays)
Between radiation induced malformations and radiation induce cancer which is a deterministic or stochastic risk
radiation induced malformation = deterministic risk
radiation induced cancer = stochastic risk
When can communication of risks associated with pregnancy be negligible?
if foetal does is less than 1mGy
What risks comes with giving pregnant lady iodine-131 and how
- crosses placenta
- foetal thyroid accumulates iodine after 10 weeks
- high foetal thyroid dose can result in permanent hypothyroidism (thyroid gland doesnt produce enough hormones)
What must be done if woman is discovered as pregnant after being given I-131 within 12 hrs
oral administration of stable potassium iodine
How does a TLD work?
- radiation absorbed
- electrons stimulated into higher energy state
- dosimeter is heated
- electrons drop out of higher energy state releasing energy as light photons
- light is equivalent to dose
Define EPD
electronic personal dosimeter
How does an EPD work
- incident radiation creates electron-hole pairs
- instant red out
number of holes is equivalent to dose
What body part I? Hp(10), Hp(0.07) and Hp(3) used for to measure dose:
Hp 10= whole body (effective dose) (measures at 10mm)
Hp 0.07= extremities skin dose (measures at 0.07)
Hp 3= eye ( measures at 3mm)
