Layered Shielding Design for Pet Clinics

The number of Positron Emission Tomography (PET) centers has been growing rapidly. Many of the new facilities have been retrofitted into existing imaging centers and hospitals. Space in the facilities is often cramped, resulting in the hot lab, patient quiet rooms, and the scanners frequently being placed in close proximity to uncontrolled areas where non-occupational dose limits apply. Of particular concern are ceilings when occupied areas are above the PET clinics. Ceiling shielding is generally constructed of layers of suspended lead, steel, and the existing concrete floor of the deck above. The attenuation provided the layered shield is difficult to calculate by point kernel techniques, yet the cost of overshielding can be high when lead must be suspended from the floor above. In this work we evaluate ceiling shielding for a proposed PET center using different methods. A Monte Carlo calculation using MCNP modeled the patient with a sitting MIRD anthropomorphic model in which 15 mCi of FDG is distributed equally among the brain and 18 bladder. A point kernel calculation was made using Mercurad, a code designed to manage attenuation in layered shields. The results from the two models are compared to each other and to measurements conducted on the finished PET clinic.