TG108 PET and PET/CT shielding requirements Flashcards
differences between PET and SPECT (TC-99m) shielding calcs
For PET, unlike Tc-99m, decay is accounted for (due to shorter half-life of F-18 of 110 minutes, compared to 6 hour half-life of Tc-99m), patient attenuation is accounted for, and calculations are carried out to assess dose in the floors above and below (140 keV photons are generally not a concern for adjacent floors, unlike 511 keV photons for PET).
are patients modelled as point sources in PET
yes
dose rate constant of F-18
0.143 uSvm^2/MBqh
dose rate constant for Tc-99m
0.0197 uSvm^2/MBqh
lead TVL for Tc-99 m
1 mm
lead TVL for 511 keV photons
16 mm
how do you account for loss of patient activity by the time the patient arrives at the scanner?
• The activity of the patient upon arriving at the scanner is reduced due to decay by a factor of 2^(-Tu/110) = e^(-ln(2) Tu/110) where Tu is the uptake time in minutes. Also, bladder voiding prior to scan removes ~15% of the administered activity – so multiply by a factor of 0.85 to account for this loss of activity.
dose reduction factors to take into account decay over period of time
{0.91, 0.83, 0.76} for {30, 60, 90} minutes
patient dose rate that accounts for patient attenuation (for F-18)
0.092 μSv m2 MBq-1 h-1
Dose at a point d away from patient during uptake time (over a week)
D = 0.092 uSv m^2/MBqh X Nw X initial activity X uptake time X R/d^2
Nw is number of patients scanned per week
R is reduction factor
what is reduction factor
Because PET tracers have short half-lives, the total radia-tion dose received over a time period, is less than the product of the initial dose rate and time
equation for transmission factor
design goal divided by weekly dose at pt d (see other card) and also divided by occupancy factor
imaging room total weekly dose at point d from patient
0.092 uSv m^2/MBqh X Nw X initial activity X 0.85 X F X t X R/d^2
t is imaging time
R is dose reduction over imaging time t
F is uptake time decay factor
0.85 is for patient attenuation and voiding
smaller transmission factor means more or less shielding?
more- needs more thickness to achieve smaller transmission
calculations for rooms above and below for PET
o Patient is assumed to be 1 m above the floor.
o Dose rate is calculated 0.5 m above the floor for rooms above and 1.7 m above the floor for rooms below
what about Ge-68 or Cs-137 for transmission studies for atenuation compensation?
Additional dose from these transmission sources is typically considered negligible and is ignored from shielding calculations
However, if CT is used for attenuation correction study, then sources need to be accounted for in CT room shielding calcs
if a room is shielded for PET, will it need additional shielding for CT?
• Due to energy differences in PET vs CT (511 keV vs. 120-140 kVp), a room that is shielded for PET is unlikely to need additional shielding for CT.
considerations for nuc med departments- multiple machines and uptake probes
have to make sure radiation from PET does not affect SPECT count or uptake probe count-have to consider this in design and shielding
