Detector development for microPET II: a 1 mu l resolution PET scanner for small animal imaging

We are currently developing a small animal positron emission tomography (PE T) scanner with a design goal of 1 microlitre (1 mm(3)) image resolution. T he detectors consist of a 12 x 12 array of 1 x 1 x 10 mm lutetium oxyorthos ilicate (LSO) scintillator crystals coupled to a 64-channel photomultiplier tube (PMT) via 5 cm long optical fibre bundles. The optical fibre connecti on allows a high detector packing fraction despite the dead space surroundi ng the active region of the PMT. Optical fibre bundles made from different types of glass were tested for light transmission, and also their effects o n crystal identification and energy resolution, and compared to direct coup ling of the LSO arrays to the PMTs. We also investigated the effects of ext ramural absorber (EMA) in the fibre bundles. Based on these results, fibre bundles manufactured from F2 glass were selected. We built three pairs of p rototype detectors (directly coupled LSO array, fibre bundle without EMA an d fibre bundle with EMA) and measured flood histograms, energy resolution, intrinsic spatial resolution and timing resolution. The results demonstrate d an intrinsic spatial resolution (FWHM) of 1. 12 mm (directly coupled), 1. 23 mm (fibre bundle without EMA coupling) and 1.27 mm (fibre bundle with EM A coupling) using an approximately 500 mum diameter Na-22 point source. Usi ng a 330 mum outer diameter steel needle line source filled with F-18, spat ial resolution for the detector with the EMA optical fibre bundle improved to 1.05 mm. The respective timing and energy FWHM values were 1.96 ns, 21% (directly coupled), 2.20 ns, 23% (fibre bundle without EMA) and 2.99 ns, 30 % (fibre bundle with EMA). The peak-to-valley ratio in the flood histograms was better with EMA (5: 1) compared to the optical fibre bundle without EM A (2.5:1), due to the decreased optical cross-talk. In comparison to the de tectors used in our current generation microPET scanner, these detectors su bstantially improve on the spatial resolution, preserve the timing resoluti on and provide adequate energy resolution for a modem high-resolution anima l PET tomograph.