Depth encoding of point-of-interaction in thick scintillation cameras

Scintillation cameras of thick NaI(Tl) crystals without collimators have been used in the field of gamma ray astronomy, positron camera and PET. In such cases, gamma rays enter into a crystal at various angles, so that the depth of interaction causes position errors in two-dimensional position encoding. In principle, the light distribution inside the crystal depends on the depth of the light source, from which depth information may be derived. The authors introduced a position arithmetic based on maximum likelihood estimation and investigated the effect of light reflection of the surface opposite to PMT array. It turned out that depth encoding is impossible with diffusive reflection, while depth information can be obtained with mirror reflection, semi-opaque and opaque reflection at a cost of spatial resolution. The compromise must be made between the spatial resolution and the depth resolution, which may depend on the specific applications.