Geometry Optimization of a Dual-Layer Offset Detector for Use in Simultaneous PET/MR Neuroimaging

PET scanners suffer from a loss of resolution due to parallax error. Depth-of-interaction detectors, such as duallayer offset (DLO) detectors, mitigate the loss of resolution. We are designing a DLO brain dedicated PET insert that fits into the Siemens Magnetom 7T brain MR scanner. A wide range of DLO detector geometries was evaluated through Monte Carlo simulations in this paper. The total detector thickness was varied from 10 to 30 mm and for each total thickness, various layer thickness ratios and crystal widths in the range of 0.5-4 mm were examined. The effects of each detector geometry on radial mispositioning of the incident photons, detection efficiency ratio between the two layers, coincidence response functions (CRFs), system spatial resolution, and sensitivity were studied. Optimum layer thickness ratio for each total thickness is discussed. Analysis of CRFs showed that the improvement in the CRF of a DLO PET scanner with a total detector thickness of larger than 10 mm is lower for detectors with a crystal width of smaller than 2 mm than for detectors with a larger crystal width. Estimated spatial resolution and sensitivity of a PET scanner with each suggested detector geometry are also reported.