Design of patient shielding to reduce the effects of out-of-field radioactivity in 3D PET

Randoms, deadtime, and scatter from activity outside the scanner are potential sources of bias and noise in volume positron emission tomography (PET). Additional scanner shielding has been shown to reduce the effects of out-of-field activity (OFA) in brain imaging. We have undertaken the design of OFA shielding that is also effective for body imaging. The shielding consists of lead sheet that is molded around, but not touching, a subject to surround the area(s) of OFA. The effectiveness of the shielding at reducing the singles flux and scattered coincidences from OFA was studied on the GE Advance PET scanner as a function of shielding thickness and axial width, patient-shielding gap, and axial source location. The optimal design was found to be 3-6-mm thick lead sheet that extends 10-15 cm from the OFA towards the scanner with a small (1-2 cm) gap between the patient and shielding allowed. With such shielding, singles are reduced by 50-70% for source positions out to 25 cm from the edge of the axial field of view, while scatter is reduced by 65-90%.