Optimizing the performance of a PET detector using discrete GSO crystals on a continuous lightguide

The authors are designing a new detector for PET using discrete 4×4×10 mm³ GSO crystals on a continuous lightguide with 39 mm PMTs. The Light Response Function (LRF) of a detector is the amount of light received by a PMT as a function of the source position. It has to be controlled by a careful design of the lightguide in order to identify 4 mm crystals. The ideal LRF should be narrow with a linear variation over the PMT diameter. Simulations show that a 1.81 cm thick lightguide produces a narrow LRF with good crystal discrimination. However, the tails of this LRF are long. A further improvement can be achieved by using a 2.31 cm lightguide with 5 mm slots cut in its front surface. This results in a sharp edged, almost triangular, LRF. The slotted lightguide also minimizes the spatial dependence on varying depths of interaction of the gamma ray. The effect of varying slot depths was also investigated through the simulations. This was done while keeping the thickness of the lightguide continuous area constant. Experiments were performed and shown to be in general agreement with the simulations. The good spatial resolution and narrow LRF of such a detector will result in a high resolution PET scanner with good count rate capability