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When the thickness of material is determined, it should be compared with the existing composition of the
boundary, if applicable, to determine what additional shielding, if any, is required. Transmission properties of
materials under various conditions are available in Appendix C.
5.2.8 Shielding calculations in nuclear medicine
The shielding requirements for a nuclear medicine facility fall into two categories - structural shielding for the
various rooms in the department and local shielding of the radionuclide sources used. Sealed and unsealed
sources must be stored in secure and adequately shielded storage facilities. In addition, devices such as lead
vials, syringe shields, shielded safes and carrier boxes must be used to reduce radiation exposure to staff,
patients and members of the public while using, transferring and administering sources in work areas.
Protecting against radiation emitted from patients who have been administered a radionuclide must be
achieved by structural shielding and department design. Shielding requirements for the scanning area, the
waiting room, patient toilets and other areas will be determined by the number of patients imaged, the
activities administered, the radiopharmaceuticals used, the time each patient remains in each part of the
facility, and the location of the facility and its environs.
The dose rate from the patient per unit activity administered is dependent on the physical and biological
half-lifes and dose rate constant of the radionuclide administered and by attenuation in the patient. An
estimate of the dose rate at 1 m from various radionuclides and the dose rate from a patient per MBq
administered are provided in Appendix D. For example, the dose rate at 1 m from a patient injected with
99m
Tc HDP is 0.0075μSvh -1 /MBq, giving a dose rate constant for the patient of 0.0075μSvm 2 /MBqh (IPSM,
1991). The effective dose equivalent dose rate constants for positron emitters are also summarised in
Appendix D.
In shielding calculations the radiation dose at the point of interest is estimated using the initial dose rate, the
inverse square law correction for distance, the decay corrections for the reduction in activity and the exposure
time. Once the radiation dose at the point of interest is known, the shielding required is calculated using an
approach similar to that used for X‐rays, but employing the attenuation data for radionuclides from Appendix
D. For shielding calculations the decrease in the activity in the patient due to “voiding” is often ignored and
only physical decay of the radionuclide is taken into account. This will somewhat overestimate the shielding
required.
In calculating the shielding requirements for different areas in the department, allowance for the radioactive
decay of the activity administered to the patient can be treated in two components:
a. The decay in activity up to the time the “active” patient enters the area, F u
, is given by:
F = exp
u
- 0.693t
T ½
u
where t u
= Time since administration (h)
T ½
= Half life (h) for the radionuclide
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