A new phoswich detector is being developed at the Crump Institute, aiming to provide improvements in sensitivity, and spatial resolution for PET. The detector configuration is comprised of two layers of pixelated scintillator crystal arrays, a glass light guide and a light detector. The annihilation photon entrance (top) layer is a 48 × 48 array of 1.01 × 1.01 × 7 mm3 LYSO crystals. The bottom layer is a 32 × 32 array of 1.55 × 1.55 × 9 mm3 BGO crystals. A tapered, multiple-element glass lightguide is used to couple the exit end of the BGO crystal array (52 × 52 mm2) to the photosensitive area of the Position Sensitive Photomultiplier Tube (46 × 46 mm2), allowing the creation of flat panel detectors without gaps between the detector modules. Both simulations and measurements were performed to evaluate the characteristics and benefits of the proposed design. The GATE Monte Carlo simulation indicated that the total fraction of the cross layer crystal scatter (CLCS) events in singles detection mode for this detector geometry is 13.2%. The large majority of these CLCS events (10.1% out of 13.2%) deposit most of their energy in a scintillator layer other than the layer of first interaction. Identification of those CLCS events for rejection or correction may lead to improvements in data quality and imaging performance. Physical measurements with the prototype detector showed that the LYSO, BGO and CLCS events were successfully identified using the delayed charge integration (DCI) technique, with more than 95% of the LYSO and BGO crystal elements clearly resolved. The measured peak-to-valley ratios (PVR) in the flood histograms were 3.5 for LYSO and 2.0 for BGO. For LYSO, the energy resolution ranged from 9.7% to 37.0% full width at half maximum (FWHM), with a mean of 13.4 ± 4.8%. For BGO the energy resolution ranged from 16.0% to 33.9% FWHM, with a mean of 18.6 ± 3.2%. In conclusion, these results demonstrate that the proposed detector is feasible and can potentially lead to a high spatial resolution, high sensitivity and DOI PET system.

Microfluidic technologies provide an attractive platform for the synthesis of radiolabeled compounds. Visualization of radioisotopes on chip is critical for synthesis optimization and technological development. With Cerenkov imaging, beta particle emitting isotopes can be localized with a sensitive CCD camera. In order for Cerenkov imaging to also serve as a quantitative tool, it is necessary to understand how material properties relevant to Cerenkov emission, namely, index of refraction and beta particle stopping power, affect Cerenkov light output. In this report, we investigate the fundamental physical characteristics of Cerenkov photon yield at different stages of [18F]FDG synthesis on the electrowetting on dielectric (EWOD) microfluidic platform. We also demonstrate how Cerenkov imaging has enabled synthesis optimization. Geant4, a Monte Carlo program applied extensively in high energy physics, is used to simulate Cerenkov photon yield from 18F beta particles traversing materials of interest during [18F]FDG synthesis on chip. Our simulations show that the majority (approximately two-thirds) of the 18F beta particle energy available to produce Cerenkov photons is deposited on the glass plates of the EWOD chip. This result suggests the possibility of using a single calibration factor to convert Cerenkov signal to radioactivity, independent of droplet composition. We validate our simulations with a controlled measurement examining varying ratios of [18O]H2O, dimethyl sulfoxide (DMSO), and acetonitrile (MeCN), and find a consistent calibration independent of solvent composition. However, the calibration factor may underestimate the radioactivity in actual synthesis due to discoloration of the droplet during certain steps of probe synthesis. In addition to the attractive quantitative potential of Cerenkov imaging, this imaging strategy provides indispensable qualitative data to guide synthesis optimization. We are able to use this imaging technique to optimize the mixing protocol as well as identify and correct for loss of radioactivity due to the migration of radioactive vapor outside of the EWOD heater, enabling an overall increase in the crude radiochemical yield from 50±3% (n=3) to 72±13% (n=5).