PSPMT/APD Hybrid DOI Detectors for the PET Component of a Dedicated Breast PET/CT System—A Feasibility Study

We explore two depth of interaction (DOI) capable detectors with a high effective packing fraction to be used for the PET portion of a next generation cylindrical pendant breast PET/CT scanner. The proposed dual-readout detectors consist of an unpolished (14 $\times$ 14) array of $(1.5\times 1.5\times 20)\ {\hbox {mm}}^{3}$ LSO crystals coupled to a position sensitive-PMT (PSPMT) at the back end for position sensing and either (A) a (2 $\times$ 2) matrix of avalanche photodiodes (APDs) or (B) a single large APD at the front end for measuring DOI. Increased effective packing fraction is achieved by choosing the APDs such that the total front surface contact area of the APD matrix or the single large APD approximately matches both the contact area of the crystal array and the active area of the PSPMT. This results in reduced dead space between adjacent detectors when placed in a cylindrical scanner geometry. We propose to recover DOI for the crystals that are partially occluded due to the inactive area of the APD by using the optical crosstalk from neighboring crystals. We conducted preliminary studies utilizing five prototype modules: two of which used a (9 $\times$ 9) array of $(1.5\times 1.5\times 20)\ {\hbox {mm}}^{3}$ LSO crystals, and three that used a (14 $\times$ 14) LSO array of $(1.5\times 1.5\times 30)\ {\hbox {mm}}^{3}$ crystals. The readout was performed either by a $(8\times 8)\ {\hbox {mm}}^{2}$, $(14\times 14)\ {\hbox {mm}}^{2}$ or a $(28\times 28)\ {\hbox {mm}}^{2}$ position sensitive-APD (PSAPD) at the front end and a PSPMT at the rear end. At room temperature, the average energy and DOI resolutions degraded only by $≪$2% and $≪$ 0.3 mm respectively for edge crystals compared to the center crystals for all PSAPDs. The energy resolution and the DOI resolution for the detector design with readout (A) was estimated to be 14.2% and 4.0 mm, and that for the design with readout (B) was estimated at 14.7% and 4.4 mm. After cooling to 10$\,\,^{\circ}{\hbox {C}}$, the performance of the second design was close to that of the first design. We conclude that a dedicated cylindrical breast PET/CT scanner could be operated with either of the proposed DOI detector schemes, thus improving packing fraction, and hence, sensitivity and NEC. The two designs have different advantages and drawbacks: the first design performs well at room temperature, however, the readout is complex, while the second design has a simpler readout scheme but requires a cooling system for it to achieve optimal performance.