Factors Affecting Hamamatsu H8500 Flat Panel PMT Calibration for Gamma Ray Imaging

The Hamamatsu H8500 Flat Panel PMT is the latest technological advancement in gamma ray imaging. Its compact size makes it attractive for medical imaging applications. To study and compare image performance a Flat Panel PMT, representing the present production, was coupled to CsI(Tl) and Nal(Tl) scintillation arrays with 3 mm and 1.8 mm pixel size respectively and connected to a multi-anode electronic readout. Furthermore, a pulsed blue LED coupled to an optical liber was utilized to scan the tube with different light distributions. This study investigated how PMT anode gain uniformity response, scintillation light distribution and intensity influence the spatial resolution, the position linearity and the image noise. Each crystal in the scintillation array produces a different charge distribution, which depends on the match between the anode size and the scintillation array lattice. The Nal(Tl) array demonstrated to fit both PMT characteristics and dynamic range of electronic read out, due to the charge distribution adequate to the anode size. For CsI(Tl) crystal, the pulse height calibration resulted more critical, due to the narrow light distribution. In conclusion, the use of Flat Panel tube with selected anode gain uniformity could represent the cheapest and easiest solution to obtain the best image quality, in particular for scintillator array with smaller pixel size.