FPGA electronics for OPET: A dual-modality optical and positron emission tomograph

The development of a prototype dual-modality optical and PET (OPET) small animal imaging tomograph is underway in the Crump Institute for Molecular Imaging at the University of California Los Angeles. OPET consists of a single ring of six detector modules with a diameter of 3.5 cm. Each detector has an 8 times 8 array of optically isolated BGO scintillators which are coupled to multichannel photomultiplier tubes and open on the front end. The system operates in either PET or optical mode and reconstructs the data sets as 3D tomograms. The detectors are capable of detecting both annihilation events (511 keV) from PET tracers as well as single photon events (SPEs) (2-3 eV) from bioluminescence. Detector channels are readout using a custom multiplex readout scheme and then filtered in analog circuitry using either a gamma-ray or SPE specific filter. Shaped pulses are sent to a digital signal processing (DSP) unit for event processing. The DSP unit has 100 MHz analog-to-digital converters on the front-end which send digitized samples to field programmable gate arrays which are programmed via user configurable algorithms to process PET coincidence events or bioluminescence SPEs. Information determined using DSP includes: event timing, energy determination-discrimination, position determination-lookup, and coincidence processing. Coincidence or SPE events are recorded to an external disk and minimal post processing is required prior to image reconstruction. Initial results from a two-detector version of OPET are shown for a PET phantom image reconstruction as well as for an optical mode acquisition. Projection images of an optical pattern placed on the front end of the detector module in the vicinity of a SPE source and positron emitting source are shown. Current work includes assembly of the full ring system and tomographic reconstruction of an optical source.