Annihilation γ-ray background characterization and rejection for a small beta camera used for tumor localization during surgery

The authors have developed a miniature (1.2 cm²) beta-ray camera prototype to assist a surgeon in locating the margins of a resected tumor. With this technique, one directly detects betas emitted from exposed radio-labeled tissue. 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 the tumor specificity of the radiopharmaceutical. The authors have characterized the effects that annihilation gamma rays have on positron imaging with their small camera. They 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 head/tumor activity, a background gamma rate of 2.0 cts/sec/μCi was measured in the CaF₂(Eu) detector. The authors present two gamma-ray background rejection schemes that require a β-γ coincidence. The first configuration uses a high efficiency scintillator coincidence “shield”, the second, a “phoswich”. Results show that these coincidence methods work with ~99% gamma ray rejection efficiency