Initial study of an asymmetric PET system dedicated to breast cancer imaging

We are developing a positron emission tomography (PET) system dedicated to breast cancer imaging and based on a pseudo-pinhole PET geometry. The system consists of a small half-ring (r=153 mm) of high-resolution LSO detectors and a large half-ring (R=413 mm) of medium-resolution LSO detectors. The coincidence detection profile between a small LSO crystal and a large LSO crystal reveals that, when the object imaged is located close to the small detector, the image resolution is primarily determined by the intrinsic spatial resolution of the small detector, and is only slightly affected by the resolution of the large detector. The dimensions of the crystals are 1.6×1.6×20 mm³ for the small half-ring and 4.3×4.3×25 mm³ for the large half-ring. Monte Carlo simulation of the system was performed using a modified SimSET package. The detectors in the small half ring are modeled as 2- or 4-layer detectors with DOI capability. One-mm point sources were simulated to measure the image resolution of the system at 0, 2, 6, and 10 cm offset along the +X, +Y and -Y directions. Results suggest image resolutions of 1.7-2.5 mm FWHM for breast tissues and 1.8-3.1 mm FWHM for tissues near the chest wall, including internal and axillary lymph nodes. Sensitivity of the system ranges from 5.4% to 7.8% and can be significantly increased if two planar detectors are attached to the system along the axial direction.