Extra shielding for improved signal-to-noise in 3D whole-body PET

In 3D PET measurements, activity outside of the direct FOV is known to degrade signal-to-noise within the direct FOV, primarily by increasing the overall rate of random coincidences. In 3D brain studies, various configurations of additional shielding around the patient have already been successfully implemented to limit the singles field of view, and thereby reduce the randoms measured by the tomograph. In this work, we extend this idea for use in the torso with a shielding configuration consisting of two "clam-shell" shields of lead surrounding the patient both above and below the axial FOV. The lead is 6 mm thick by 10 or 20 cm in axial length, and curved into a C-shape to fit around the phantom or the patient's torso. The top half of the clam-shell rests on plastic wheels which travel on two rails mounted to the edge of the patient pallet, and the lower half of each shield is fixed beneath. The shields are placed just outside the direct field of view and as close to the patient as possible. Noise Equivalent Count (NEC) curves were measured with and without shields in place around an axially long cylindrical phantom. The NEC rates from 2D and 3D patient studies were then compared with the phantom-derived NEC curves to predict the signal-to-noise gain possible by using the shields. Over the range of activity concentrations typical of whole body FDG studies, we estimate unshielded 3D offers an NEC advantage of approximately a factor of two. Our shield appears to provide only a small additional improvement at the high end of this activity range.