Uniformity testing of a dual-head gamma camera with a nearby point source

Limitations to head movement of dual-head gamma cameras may prevent proper geometry being attained for testing of intrinsic uniformity with a point source. When a point source is placed too near to the uncollimated crystal, the detected flood image will be non-uniform due to the inverse square law effect, oblique incidence, and increased path length traversed through the crystal by oblique rays. A formula has been deduced for constructing a map to correct for these effects. The general formula has been applied to a particular dual-head camera in which a point source has been placed on the face of one head, at 65.5 cm from the opposite uncollimated crystal. It is shown that division of the non-uniform image, acquired with this geometry, by the correction map yields a uniform image which may be used for routine investigation of the uniformity characteristics of the camera.