A High Resolution Detection System for Positron Tomography

The feasibility of using a hybrid detection system, consisting of multiple narrow bismuth germanate (BGO) crystals coupled to one photomultiplier for coincidence timing, and individual mercuric iodide (HgI2) photodetectors coupled to each BGO crystal to identify crystals of interaction, was investigated for application to positron tomography. The match between the emission spectrum of BGO and the photoresponse spectrum of HgI2 in combination with recently developed HgI2 x-ray detector and low noise preamplifier technology provided the appropriate high sensitivity, low noise environment required to detect the low levels of scintillation light produced in the absorption of 511 keV gamma-rays in BGO. Energy resoluton of 24% for 511 keV was obtained with the BGO-HgI2 detector, compared to 18% for BGO with a PMT. Coincidence resolving time for the HgI2-BGO detector was measured to be 105 nanoseconds. This performance of a non-optimized detector system is nominally adequate for appropriately designed tomographs.