Simultaneous PET and MR imaging with a 7-T system was deemed feasible.

No abstract

Avalanche photodiodes (APDs) have proven to be useful as light detectors for high resolution positron emission tomography (PET). Their compactness makes these devices excellent candidates for replacing bulky photomultiplier tubes (PMTs) in PET systems where space limitations are an issue. The readout of densely packed, 10 x 10 lutetium oxyorthosilicate (LSO) block detectors (crystal size 2.0 x 2.0 x 12 mm3) with custom-built monolithic 3 x 3 APD arrays was investigated. The APDs had a 5 x 5 mm2 active surface and were arranged on a 6.25 mm pitch. The dead space on the edges of the array was 1.25 mm. The APDs were operated at a bias voltage of approximately 380 V for a gain of 100 and a dark current of 10 nA per APD. The standard deviation in gain between the APDs in the array ranged from 1.8 to 6.5% as the gain was varied from 50 to 108. A fast, low-noise, multi-channel charge sensitive preamplifier application-specific integrated circuit (ASIC) was developed for the APD readout. The amplifier had a rise time of 8 ns, a noise floor of 515 e- rms and a 9 e- pF(-1) noise slope. An acquired flood image showed that all 100 crystals from the block detector could be resolved. Timing measurements with single-channel LSO-APD detectors, as well as with the array, against a plastic scintillator and PMT assembly showed a time resolution of 1.2 ns and 2.5 ns, respectively. The energy resolution measured with a single 4.0 x 4.0 x 10 mm3 LSO crystal, wrapped in four-layer polytetrafluoroethylene (PTFE) tape and coupled with optical grease on a single APD of the array, yielded 15% (full width at half maximum, FWHM) at 511 keV. Stability tests over 9 months of operation showed that the APD arrays do not degrade appreciably. These results demonstrate the ability to decode densely packed LSO scintillation blocks with compact APD arrays. The good timing and energy resolution makes these detectors suitable for high resolution PET.

A flexible, extensible, high-speed architecture, called QuickSilver TM and specifically geared to the requirements of small animal imaging, has been developed. The architecture is composed of ring-based event processing modules (EPMs) with nearest neighbor, high-speed digital communication transmitting event packets via a "store and forward" concept. Each EPM is capable of transmitting up to 15.6M events/sec to other EPMs. Coincidence determination is performed at the EPM level around the "ring". This distributes the load and eliminates the need for a separate coincidence processor. Each EPM is capable of transmitting up to 1.9M coincidence events/sec to an event routing subsystem (ERS) for acquisition and processing. The ERS has 2 transport interfaces for acquiring events: an IEEE 1394A interface and a PCI interface. The IEEE 1394A interface can support up to 5.3M events/sec and the PCI interface can support up to 16.7M events/sec. Thus this architecture provides a new level of capability for small animal PET imaging, but is also extremely well suited for PET research, single photon emission computed tomography (SPECT) imaging, and use with X-ray CT and magnetic resonance imaging (MRI).