Development and Comparison of DOI capable Detectors for a Dedicated Breast PET/MRI Insert

Our group is developing a PET/MRI insert for enhanced breast imaging with a clinical 3T whole-body PET/MRI scanner. The small bore-diameter in combination with long, thin scintillation crystals promotes parallax error, which we aim to mitigate by determining the depth of interaction (DOI) for incident gammas. We therefore developed and compared two different DOI capable detectors based on a modified version of the C13500 TOF-PET modules (Hamamatsu Photonics K.K). A $4 \times 450 \mu \mathrm{m}$ S13546H MPPC array (Hamamatsu Photonics K.K.) was coupled to the following LSO crystal blocks separated by ESR and a 1.6 mm crystal pitch: a $10 \times 10$ crystal 15 mm high single layer block or a dual layer block with $10 \times 10$ crystals of 9.8 mm height (bottom layer) and $9 \times 9$ of 5.2 mm height (top layer). DOI information was extracted from the light spread pattern for the single layer block using a $380 \mu \mathrm{m}$ top light guide. DOI profiles were obtained using a tungsten collimated coincidence setup with defined irradiation depth at $3 \mathrm{~mm}, 8 \mathrm{~mm}$ and 13 mm with a 1.2 GBq F-18 line source. Both configurations showed a good crystal identification (peak to valley ratio of $7.3 \pm 3.3$ (single layer) and $4.3 \pm 1.6$ (dual layer)). Mean energy resolutions of $13.6 \pm 2.2 \%$ (single layer) and 13.3 $\pm 2.0 \% / 14.3 \pm 3.8 \%$ (dual-layer bottom/top layer) were achieved. The DOI resolution for the dual layer block inherently was limited to two DOI bins, with the single layer a DOI resolution of $5.3 \pm 2 \mathrm{~mm}$ was obtained. These preliminary evaluations demonstrate the suitability of both configurations for the breast insert and currently we prepare a timing related comparison. The higher DOI resolution of the single layer configuration requires a detector individual DOI calibration. Therefore, we are currently setting up a ring configuration with 8 detector modules and precise automated translational stages to develop system-applicable DOI calibration methods.