This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We are developing a new type of PET (positron emission tomography) device for breast imaging applications that uses a high-resolution semi-circular ring of detectors within a conventional PET/CT scanner to zoom-in on the region around a breast to obtain high-resolution images. To aid the development of our system, we must perform realistic Monte Carlo simulations to understand the performance of our system and to generate a system matrix. A system matrix represents the probability of detecting a pair of gamma rays originated from a voxel in the imaging field of view. One can use the system matrix in a statistical iterative reconstruction framework to estimate the distribution of radioactivity in the patient. We have evaluated existing Monte Carlo simulation packages such as GATE, a Geant4-based simulation program, to perform this simulation study, but found that none of the existing software will handle our unique system geometry efficiently. We are developing our own simulation program that specifically models our geometry in an efficient fashion. We would like to evaluate the accuracy of our simulation program, and once done, generate the full system matrix, and perform image reconstruction using the Teragrid computing resources. To achieve these goals, we anticipate that we will need a full 30,000 SU development allocation.