Validation of the spatially variant probability density functions for the HRRT

The probability density function (PDF) is a new concept of modeling the spatial dependence of the system response. In iterative reconstruction algorithms, the PDF plays the same important role as the point spread function in recovering images of high resolution. For the HRRT with a large number of lines of response, Monte Carlo simulation, along with an effective parameterization strategy, provides a practical solution to generating the PDFs that can be used in our listmode reconstruction algorithm (MOLAR). However, the PDF's accuracy, which depends on the accuracy of the simulated HRRT scanner, is still unverified. This study aims to validate the PDFs derived from simulation for the HRRT, by comparison to binned point source data. It is shown that the shape and width of the PDFs vary as a function of the incident angles to the detectors, and depth along each LOR, consistent with simulation results. However, the variability by detector layer for the HRRT phoswich is found to be significant in the experimental results, with roughly 20-40% degradations in FWHM and FWTM between the front-layer and back-layer PDFs, as compare to simulated results. On the other hand, all experimentally derived PDFs have larger spread than the simulated PDFs. The differences in FWHM, which are layer-dependent, range from 0.5mm to 1.5mm. We conclude that corrections of simulated PDF are required in order to achieve accurate resolution modeling.