Detector response function of the NanoPET™/CT system

Even with the huge advance of computers and computing power, full three dimensional reconstructions of PET and CT scans belong to the future as far as commercially available scanners and software are concerned. Recent investigations have shown that with the aid of Graphical Processing Units (GPUs) extremely high computational speed might be achieved, which lends itself to the implementation of iterative 3D reconstruction techniques. Moreover, these techniques make it possible to make use of off-line and on-the-fly Monte Carlo calculations. A consortium of several Hungarian institutions has been working on the development and optimization of a Monte Carlo supported 3D iterative reconstruction program. The in-body scatter processes are modeled by real-time Monte Carlo, however, the detector response is calculated off-line. Therefore, the effective implementation of this MC code requires the calculation of a detector response function in advance. The paper describes our analysis of the response function characteristics of the NanoPET™ (Mediso) detector system. Using MCNPX we constructed a data base consisting of 300 simulations (different incoming photon angles and energies). We studied the sensitivity of the system to several parameters. It was found that the spatial dependence is stronger than the energy dependence. At right angle (at 511 keV) the side-neighbors have an order of magnitude less probability compared to the central pixel, while for photons reaching the central pixel with 350 keV or 511 keV there is only 40% difference in the probabilities. We studied different techniques to include the response function into the MC code mentioned in order to find the optimal strategy. With the approach described in the paper a significant improvement in image quality can be obtained.