Annihilation gamma ray background characterization and rejection for a positron camera

We have developed a miniature (1.2 cm²) beta-ray camera prototype to assist a surgeon in locating and removing the margins of a resected tumor. When imaging positron emitting radiopharmaceuticals, annihilation gamma ray interactions in the detector can mimic those of the betas. The extent of the background contamination depends on the detector, geometry and tumor specificity of the radiopharmaceutical. We have characterized the effects that annihilation gamma rays have on positron imaging with the 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 ¹⁸F to simulate annihilation background from the brain. For various ratios of phantom brain/tumor activity, a annihilation gamma rate of 1.8 cts/sec/μCi was measured in the CaF ₂(Eu) detector. We present two gamma-ray background rejection schemes that use a β-γ coincidence. Results show that the coincidence methods works with ~99% gamma ray rejection efficiency