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Hybrid imaging systems, comprising PET and Compton camera modules, have recently gained in interest, due to their capability to simultaneously detect positron annihilation photons and γ-rays from single-photon emitting sources as also used in SPECT. A unique feature of such systems, however, is the capability to also be operated in a so called γ-PET mode. Here, specific β+- emitting radioisotopes (such as 44Sc, 1°C or 14O) are used to detect triple-coincidences between two annihilation γ-rays (in PET imaging) and a third, prompt photon (in Compton imaging), that is emitted by the deexcitation of the decay’s daughter nucleus. Consequently, an intersection between the line-of-response (LOR) and the Compton cone can be determined, which (in principle) allows to localize the photons’ emission vertices on a single decay basis. In practice, however, a few tens of events are required to localize a point source, which still results in a considerable sensitivity improvement compared to conventional PET imaging.For a proof-of-principle study, we used a pixelated GAGG crystal array (16 × 16 crystals; 1.45 × 1.45 × 6 mm3 crystal volume; 25 μm SPAD SiPMs as readout) as Compton camera scatterer and PET detectors, and a three-layered LYSO crystal array (1.2 × 1.2 × 6.66 mm3 crystal volume; 50 μm SPAD SiPMs as readout) as Compton camera absorber. We characterized the individual detector components with regard to their energy resolution and the capability to identify the various scintillator array’s individual crystals. Our first γ-PET prototype was tested in PET-only and Compton-only imaging mode, in which spatial resolutions of 3.2–3.5 mm FWHM (PET-only mode) and 14.4–19.3 mm FWHM (Compton-only mode at 1,274 keV) were achieved, respectively, using a22Na point source and 10 iterations of an ML-EM reconstruction algorithm. By using triple-coincidences in a γ-PET mode (event-wise intersection of the LOR and the Compton cone), we could demonstrate the capability of the prototype to perform a full 3D point source reconstruction using only 77 events.

Abstract: Squamous cell carcinoma (SCC) is the most common cancer of the head and neck (HNC). Advanced HNC warrants extensive ablative and reconstructive procedures, significantly altering locoregional anatomy, while radiation treatment further adds to the distortion. Anatomic imaging is therefore often inconclusive in suspected recurrent HNC. Functional imaging with fluorodeoxyglucose-positron emission tomography (FDG-PET) has been reported contributory in the evaluation of patients with SCC. While most reports are from dedicated PET systems, full ring PET is of limited availability and gamma-PET may offer a suitable compromise. The therapeutic impact of gamma-PET in patients with suspected recurrent HNC was retrospectively evaluated. Seventeen patients were evaluated. All had undergone surgery for HNC, 16 also received radiotherapy. gamma-PET scans were compared to anatomic imaging, histopathology and clinical follow-up. The impact of the FDG-PET scan on patient management was then evaluated. Eleven positive findings were confirmed. Two false positives were due to radiation changes, a recognized pitfall. There were no false negatives. Overall accuracy of the 18F-FDG gamma-PET scans was 88% with considerable effect on patient management. Gamma-PET with FDG appears valuable in the evaluation of suspected recurrent HNC, and may provide a suitable alternative when dedicated PET is unavailable. [Copyright &y& Elsevier]