TG108 PET and PET/CT shielding requirements

Question 1

differences between PET and SPECT (TC-99m) shielding calcs

Answer 1

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).

Question 2

are patients modelled as point sources in PET

Answer 2

yes

Question 3

dose rate constant of F-18

Answer 3

0.143 uSvm^2/MBqh

Question 4

dose rate constant for Tc-99m

Answer 4

0.0197 uSvm^2/MBqh

Question 5

lead TVL for Tc-99 m

Answer 5

1 mm

Question 6

lead TVL for 511 keV photons

Answer 6

16 mm

Question 7

how do you account for loss of patient activity by the time the patient arrives at the scanner?

Answer 7

• 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.

Question 8

dose reduction factors to take into account decay over period of time

Answer 8

{0.91, 0.83, 0.76} for {30, 60, 90} minutes

Question 9

patient dose rate that accounts for patient attenuation (for F-18)

Answer 9

0.092 μSv m2 MBq-1 h-1

Question 10

Dose at a point d away from patient during uptake time (over a week)

Answer 10

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

Question 11

what is reduction factor

Answer 11

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

Question 12

equation for transmission factor

Answer 12

design goal divided by weekly dose at pt d (see other card) and also divided by occupancy factor

Question 13

imaging room total weekly dose at point d from patient

Answer 13

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

Question 14

smaller transmission factor means more or less shielding?

Answer 14

more- needs more thickness to achieve smaller transmission

Question 15

calculations for rooms above and below for PET

Answer 15

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

Question 16

what about Ge-68 or Cs-137 for transmission studies for atenuation compensation?

Answer 16

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

Question 17

if a room is shielded for PET, will it need additional shielding for CT?

Answer 17

• 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.

Question 18

considerations for nuc med departments- multiple machines and uptake probes

Answer 18

have to make sure radiation from PET does not affect SPECT count or uptake probe count-have to consider this in design and shielding