Design and fabrication of an electronic instrument for testing and characterization of photomultiplier tubes to improve the image quality of LFOV Siemens Gamma Camera

Abstract

A photomultiplier tube (PMT) electronic instrument was designed and fabricated. High voltage (-2000 V) and low voltage supplies (+8, ±12 V) were also designed and constructed for the instrument. 40 PMTs were tested for voltage gains to determine whether they were good or bad using photoelectric effect principle. Good and bad PMTs were identified during the measurement of their gains. The dark currents of the PMTs were measured with a Keithy Electrometer and PMTs with high dark current (10k Ix jJA) were declared bad alongside the bad PMTs and were rejected. PMTs that were identified as good during the testing were then sorted by their gains and arranged on the NaI (Tl) detector of the gamma camera. The PMT with the highest gain was placed at the centre, and PMTs with the lowest gains were placed at the outer circle (edge) ofthe detector. The power supplies and the signal processing boards of the gamma camera were then aligned (calibrated) to bring their offset voltage to zero (±O.OOI V) and the PMTs HV pots were adjusted to bring their individual energy to the same photo-peak (140 Kev of the 99mTe radionuclide). Quality control pictures which were taken after alignment and fine-tuning the PMTs showed some high improvement of image quality. The central field of view (CFOV) Integral Uniformity was 1.64 %; and the Differential Uniformity was 1.13 %. The Useful field of view (UFOV) Integral and Differential Uniformities were 1.67 % and 1.26 %, respectivelyThe results compared to NEMA showed that the Gamma camera uniformity and linearity were highly improved