ANNIHILATION GAMMA-RAY BACKGROUND CHARACTERIZATION AND REJECTION FOR A SMALL BETA-CAMERA USED FOR TUMOR-LOCALIZATION DURING SURGERY

We have developed a miniature (1.2 cm(2)) beta-ray camera prototype to assist a surgeon in locating the margins of a resected tumor. With th is technique, one directly detects betas emitted from exposed radio-la beled tissue. When imaging positron emitting radiopharmaceuticals, ann ihilation gamma ray interactions in the detector can mimic those of th e betas. The extent of the background contamination depends on the det ector, geometry and the tumor specificity of the radiopharmaceutical. We have characterized the effects that annihilation gamma rays have on positron imaging with our small camera. We studied beta and gamma ray detection rates and imaging using small positron or electron sources directly exposed to the detector to simulate hot tumor remnants, and a cylinder filled with F-18 to simulate annihilation background from th e brain. For various ratios of phantom head/tumor activity, a backgrou nd gamma rate of 2.0 cts/sec/mu Ci was measured in the CaF2(Eu) detect or. We present two gamma-ray background rejection schemes that require a beta-gamma coincidence. The first configuration uses a high efficie ncy scintillator coincidence ''shield'', the second, a ''phoswich''. R esults show that these coincidence methods work with similar to 99% ga mma ray rejection efficiency.