Improvement of Image Quality in Multizone Scintillation Cameras Using Fiber-Optic Coupling to Reduce Effective PMT Size

Abstract

Earlier papers by the author showed that the resolution and sensitivity of a multizone scintillation detector depend not only on the thickness of the scintillator but also on the number of detector zones, and hence the photomultiplier tube (PMT) array size. Standard design practice is to couple the PMTs directly to the scintillator surface, which means that the maximum array size is determined by the smallest PMT size available. An increase in array size, however, leads to reduction in the PMT packing fraction due to the glass walls. This paper shows that effective PMT size can be reduced as much as desired by coupling a given scintillator area to a relatively larger PMT surface area via optical fibers of suitable bandwidth for the scintillation light. Fiber-optic coupling not only increases the packing fraction to 100% but also enables the PMTs to be chosen based entirely on performance and/or cost without regard to shape and size. While low spatial resolution and image contrast is characteristic of nuclear imaging systems in general, rapid high-count-rate quantitative studies are particularly subject to severe distortion due to dead-time losses despite efforts to minimize and/or correct for these losses. An increase in the number of detector zones via fiber-optic coupling enables significantly higher resolution and sensitivity to be achieved, more accurate sampling of the light distribution to be obtained, and detector operation to be virtually linear so that dead-time losses are practically insignificant. Statistical noise in nuclear images arises from variability in the number of light photons emitted per scintillation event, differences in the quantum efficiencies of the PMTs, and PMT gain fluctuations due to variations in the secondary emission ratios of dynodes. This paper also shows that an increase in array size using fiber-optic coupling reduces the effect of PMT and preamplifier gain nonlinearity, thereby leading to substantial improvement in image quality.