AbstractsBiology & Animal Science

Current challenges facing us in developing dedicated positron emission tomography (PET) systems for metabolic breast mammography (PEM) and small animal (ANIPET) are to achieve high spatial resolution (less than 2 mm) and high efficiency. It is also crucial to extend the sensitive areas of PEM detectors to their periphery in order to overcome the difficulty in imaging near a patient's chest wall. This limitation of the periphery dead region was revealed in the clinical trials of our previously developed PEM-I system. In the new study, we developed prototype detectors by using position-sensitive photomultiplier tubes (PS-PMTS) and pixelated bismuth germanate (BGO) crystals with depth encoding scheme to detect and localize gamma rays. We used the following methods in crystal processing: (1) Crystal cutting – each crystal block was cut by diamond saw into small elements of 2.1 mm x 2.1 mm (2.2 mm pitch) on two opposite faces. The elements on one face of the block offset by half the crystal pitch from those on the opposite face in both X and Y dimensions. The depths of two layers were 11.5 mm and 6.5 mm, respectively. The middle solid space between the two layers was 2 mm. (2) Crystal polishing and encapsulating – The very roughly cut surfaces were chemically polished by acid etching method and the cut slots were encapsulated with an epoxy-compound mixture. (3) Crystal separating – the pseudo discrete crystal blocks were cut along the middle solid space into two discrete segments. (4) Crystal coupling – the two crystal segments were glued together and optically coupled to the PS-PMTs window. We also developed front-end electronic circuits including high-voltage dividers, anode resistor chains, position readout circuits, and last-dynode timing circuits. Methods for combining four PS-PMTs with simple X+, X-, Y+, Y- outputs have been developed to further simplify the position recording. The detectors were constructed in the structure of arrays (two in the system) – modules (four in each array) – units (four in each module). The basic unit of one crystal and one PS-PMT was formed as field replaceable unit. The acquired list-mode data were analyzed with MATLAB and C. Different methods to generate distortion look-up-table were examined and evaluated. Our new prototype detectors have spatial resolutions of 1.8 mm (vs. 2.8 mm in PEM-I), timing resolution of 10.3 ns (vs. 12 ns in PEM-I), and a field-of-view of 88 mm x 88 mm (vs. 64 mm x 56 mm in PEM-I). Our analysis shows that the design improves the spatial resolution, enhances the detector field-of-view, and significantly reduces the peripheral dead regions.