Rad Prot practical aspects Flashcards
DAP
dose area product
used in diagnostic radiology, EXCEPT CT
starting point for estimating patient effective dose
used to estimate scattered doses from patient
used to audit practice
DLP
dose length product
used for CT
starting point for estimating patient effective dose
used to estimate scattered doses from patient
used to audit practice
DAP calc
= xray beam area x absorbed dose
DAP meter - a parallel plate ionisation chamber mounted at xray tube
independent of distance from xray tube
will increase if field size increased
Gycm^2
look up tables required to convert DAP to organ or effective doses
DLP in CT calc
= CTDIvol * L
L scanned length
no DAP due to geometry
use pencil ionisation chamber
depends on exposure factors and beam width
mGy.cm
measure dose along z axis
CT dose index CTDI
mGy
area under curve/T
CTDIvol
takes into account pitch
measured using a phantom
= CTDIw / pitch
weighted CTDI
CTDIw = 1/3 CTDIcentre + 2/3 CTDIperiphery
mGy
exposure controls
most to least important
engineering controls and design features
safety features and warning devices
systems of work
PPE
Lead PPE
0.25mm will reduce scattered dose by approx 10 times at 100kV
0.35mm: nearly 15 times
scattered beam protection
TVL
tenth value layer
thickness of the material that reduces the dose rate to 0.1 of incident dose rate
eg. 22mm lead
2 TVL
reduces dose by 100 times
annual dose limits
employees
Effective 20 mSv
equiv:
eye lens 20
skin 500
extremities 500
classified person
dose expectations
E >6mSv
eye > 15
skin or extremity >150
estimating staff doses
dosimeter tech
thermo-luminescent dosimeters (TLDs)
optically stimulated luminescent (OSL) dosimeters
variety of electronic devices
must be approved by HSE
effective dose monitoring
measures body and skin dose
TLD or OSL
eye monitors
eye dose
TLD
extremity monitors
skin dose
TLD
DIL
dose investigation level
effective dose level set by employer and given in local rules
when exceeded, a review of working conditions is required to ensure doses are ALARP
(workload, equipment…)
staff monitored
set lower than dose limit
annual level required
equivalent doses
lead aprons
increase distance by 30cm, reduce effective dose by half
thyroid shields
reduction in effective dose by > 5 times with 0.35mm Pb
eye shields
reduction in eye dose limit to 20mSv per year
dose reduction 5-10 times
shielding materials
selection
depends on
radiation type and energy
thickness and weight
permanence
cost
optical transparency
shielding materials
lead sheet
lead glass and lead acrylic
concrete
brick