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

shielding for 18 F procedures should be more than adequate for procedures where similar quantities of
radioactivity of shorter-lived radionuclides e.g. 11 C, 13 N, 15 O, 82 Rb or those with smaller dose rate constants
e.g. 64 Cu, 68 Ga are administered. However, it should be noted that radionuclides that have higher energy
gamma emissions, in addition to annihilation radiation, might not be adequately shielded in a facility designed
for 18 F (AAPM, 2006).
Radioactivity administration & uptake
The amount of activity administered for 18 F FDG studies is determined by the mass of the patient, the length
of the uptake time, and the acquisition mode. Shielding calculations for a facility must therefore take account
of the maximum activity to be administered in the facility.
Factors affecting dose rates from PET/CT patients
Following administration of the radionuclide, the patient is the primary source of radiation. In determining
the radiation dose in areas around the patient, or from an unshielded source, the following points must be
considered:
Dose rate constant
When calculating the shielding requirements for an unshielded 18 F source, a dose rate constant of 0.143
μSvm 2 /MBqh should be used. The dose rate from an unshielded 37 MBq 18 F point source is
5.3 μSv/h at 1 m (AAPM, 2006).
Attenuation by patient & scanner
Attenuation by patient
Since the body absorbs some of the annihilation radiation, the dose rate from the patient is reduced by a
significant factor. The AAPM recommends using a dose rate of 0.092 μSvm 2 /MBqh from the patient
immediately after administration. This corresponds to an effective body absorption factor of 0.36.
Attenuation by scanner
The gantry and detectors can provide shielding which may substantially reduce the dose rate at some of the
walls of the imaging room (AAPM, 2006). This reduction depends on the layout of the room, the shielding
characteristics of the scanner and the type of scanning procedures. Detailed knowledge of these is required
to accurately estimate the dose reduction that can be achieved. The AAPM report estimates an average dose
reduction of 20% which is partially offset by the exposure during the time the patient is being brought into
the room and positioned on the table. They suggest a figure of 15% overall reduction as being more realistic.
The most conservative approach to shielding requirements will ignore this potential dose reduction.
Physical decay
Decay during uptake or imaging
Because PET radionuclides have short half-lives, the total radiation dose received over a time period t, D(t), is
less than the product of the initial dose rate and time [Ḋ(0) x t]. The reduction factor, R t
is calculated as
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