Large surface gamma cameras for medical imaging: characterization of the bismuth germanate blocks

The CLaRyS collaboration focuses on the development of gamma detectors for medical applications, in particular for what concerns the range monitoring in ion beam therapy. Part of the research program aims to implement two gamma-camera clinical prototypes, a multi-collimated camera and a Compton camera. A common absorber detector has been designed for the two prototypes, based on bismuth germanate (BGO) blocks, 3.5x3.8x3 cm(3), assembled in various geometrical configurations to meet the application requirements. The surface of each block is streaked in a matrix of 8x8 pseudo pixels, which makes possible a position reconstruction via Anger logic from the signals collected by four read-out photo-multiplier tubes. The whole set of available blocks comes from a dismantled positron emission tomography system by Siemens, so that each single block must be tested and characterized in terms of space, time and energy response. We present in this work the implemented characterization method, which leads to a complete estimation of the block response via gamma source irradiations and data analysis devoted to a custom calibration for the imaging performance optimization of each detector module. A reference set of blocks has been completely characterized and showed very homogeneous responses: the average energy resolution is 25% FWHM at 511 keV and 20% FWHM at 1275 keV, the time resolution ranges between 3.9 and 5.3 ns FWHM and the spatial resolution has been verified to be limited to the pseudo-pixel size.