How crystal configuration affects the position detection accuracy in pixelated molecular SPECT imaging systems?

Abstract

It is well known that inter-crystal scattering and penetration (ICS–P) are major spatial resolution limiting parameters in dedicated SPECT scanners with pixelated crystal. In this study, the effect of ICS–P on crystal identification in different crystal configurations was evaluated using GATE Monte Carlo simulation. A 99mTc pencil-beam toward central crystal element was utilized. Beam incident angle was assumed to vary from 0° to 45° in 5° steps. The effects of various crystal configurations such as pixel-size, pixel-gap, and crystal material were studied. The influence of photon energy on the crystal identification (CI) was also investigated. Position detection accuracy (PDA) was defined as a factor indicating performance of the crystal. Furthermore, a set of 99mTc point-source simulations was performed in order to calculate peak-to-valley (PVR) ratio for each configuration. The results show that the CsI(Na) manifests higher PDA than NaI(Tl) and YAP(Ce). In addition, as the incident angle increases, the crystal becomes less accurate in positioning of the events. Beyond a crystal-dependent critical angle, the PDA monotonically reduces. The PDA reaches 0.44 for the CsI(Na) at 45° beam angle. The PDAs obtained by the point-source evaluation also behave the same as for the pencil-beam irradiations. In addition, the PVRs derived from flood images linearly correlate their corresponding PDAs. In conclusion, quantitative assessment of ICS–P is mandatory for scanner design and modeling the system matrix during iterative reconstruction algorithms for the purpose of resolution modeling in ultra-high-resolution SPECT.