Readout of the optical PET, (OPET) detector

The design of an imaging system able to detect both high-energy γ-rays and optical wavelength photons is underway at the Crump Institute for Molecular Imaging. This system will be capable of non-invasively and repeatedly imaging small animal models in-vivo for the presence of positron emission tomography (PET) and optical signals. The detector will consist of modules of multichannel photomultiplier tubes (MC-PMT) coupled to arrays of LSO or GSO crystals. The MC-PMT will detect both the photons produced due to bioluminescence and the photons generated in the interaction of γ-rays within the crystals. The long wavelength photons produced through bioluminescence in the animal are only slightly attenuated by these crystals and are detected directly at the photocathode of the MC-PMT. The resulting signal will consist of small (5-10 mV) short (15 ns) pulses. In contrast, the annihilation (511 keV) γ-rays that interact in the crystal will produce a large burst of photons for which the PMT output can be as large as 0.5-1 V and of >200 ns duration. The processing in a single circuit of pulses with such different characteristics requires significant alteration of the standard pulse processing circuitry used in PET scanners. In this paper we discuss the requirements of such a circuit and show the results of implementation of one design using a single channel PMT. The detector concept and OPET system are described in M7-9.