The Design of Diagnostic Medical Facilities where ... - ResearchGate

Example 7: Shielding required for PET scanning room
A PET facility scans 50 patients per week using 18 F FDG. The administered activity per scan is 555 MBq. The
uptake time is 1 hour and the average imaging time is 30 minutes.
What shielding is required for the wall of the control room which is 3 m from the scanning table
Because of the delay between administration and imaging, the activity in the patient will decrease by
F = exp
u
- 0.693t
T
1 2
u
where t u
is the uptake time(min) and T 1/2
the half-life (min). For t u
of 60 mins,
-0.693×
60
F u
= exp
= 0.69
110
In most cases, the patient will void prior to imaging, thereby removing about 15% of the administered
activity and decreasing the dose rate by a factor of 0.85. Because the 18 F FDG will decay during imaging,
a further dose reduction factor R ti
is applied
D(t)
= .
D(0)
R t i
×
t
T
= 1.443×
t
For a scanning time t of 30 mins, R ti
is equal to 0.91.
½
× 1−
exp
− 0.693t
T
½
Equation 5.8
The dose D(t i
) per imaging time at a distance 3 m from the patient is given by Equation 5.11
D(t i
) =
2
Fu
0 .092 μSvm /MBqh × A
o
(MBq) × ti
(h) × R
ti
× 0.85 ×
2
d (m)
=
0.69
0 .092×
555×
0.5 × 0.91×
0.85×
9
= 1.514 μSv
2
Scanning 50 patients per week, the annual dose at 3 m from the Table, D, is
D(t a
) = 1.514 μSv x 50 x 52 = 3936 μSv = 3.94 mSv
If the shielding characteristics of the scanner are accurately known, then a dose reduction factor at
some of the walls may be incorporated into the calculation (see Section 5.2.9). A more conservative
approach ignores the potential dose reduction afforded by the scanner.
It is assumed that only designated radiation workers will occupy the control room (dose constraint of
1 mSv) and that it has an occupancy factor of 1. The required transmission factor will be given by:
B 1mSv
=
0.25
3.936 mSv × 1
=
Using Table D.4 and Figures D.1 and D.2 from Appendix D, 10 mm lead or 13.3 cm concrete is required
to provide the necessary shielding.
The Design of Diagnostic Medical Facilities where Ionising Radiation is used 73