Positron emission tomography (PET) is a molecular imaging modality that provides information at the molecular level. This system is composed of radiation detectors to detect incoming coincident annihilation gamma photons emitted from the radiopharmaceutical injected into a patient's body and uses these data to reconstruct images. A major trend in PET instrumentation is the development of time-of-flight positron emission tomography (ToF-PET). In ToF-PET, the time information (the instant the radiation is detected) is incorporated for image reconstruction. Therefore, precise and accurate timing recording is crucial in ToF-PET. ToF-PET leads to better localization of the annihilation event and thus results in overall improvement in the signal-to-noise ratio (SNR) of the reconstructed image. Several factors affect the timing performance of ToF-PET. In this article, the background, early research and recent advances in ToF-PET instrumentation are presented. Emphasis is placed on the various types of scintillators, photodetectors and electronic circuitry for use in ToF-PET, and their impact on timing resolution is discussed.

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Depth-of-interaction (DOI) encoding can contribute to improving spatial resolution uniformity and sensitivity in positron-emission-tomography (PET) scanners. In addition, time-of-flight (TOF) PET scanners with DOI encoding have received considerable interest because of their potential for improving the spatial resolution, sensitivity, and image quality of the overall system. In this study, a new DOI detector configuration utilizing scintillators’ emission wavelength is proposed, and experimental results on the energy, timing, and DOI performance of the detector are provided. The DOI information from the proposed phoswich-type detector can be acquired at the detector level without complex signal processing by utilizing a single optical filter with customized optical properties. For this, we used either a short pass filter (SPF) or a long pass filter (LPF) that allows light photons of a specific wavelength to pass. The two-layered phoswich detector was configured with two scintillators with different photon-emission spectra. In this study, we used Ce:GAGG (3 mm  ×  3 mm  ×  10 mm) and LYSO:Ce (3 mm  ×  3 mm  ×  10 mm) as the top and bottom layer scintillators, respectively. A digital silicon photomultiplier (dSiPM) was used as the photosensor and for data acquisition. The phoswich detector was placed in the center of two dSiPM pixels, where one of the dSiPM pixels was covered with the optical filter, and the light guide was placed on the other pixel. The detector was tested for energy, timing, and DOI encoding performance. When an SPF was used, the energy resolutions of 16.2% and 11.8% were achieved for the Ce:GAGG (top layer) and LYSO:Ce (bottom layer) respectively without correcting for saturation effect. With a small (3 mm  ×  3 mm  ×  5 mm) LYSO crystal as the reference detector, CRTs (coincidence-resolving times) of 338 ps and 244 ps were recorded for the top and bottom layers respectively. The detector configuration also provides an excellent DOI-separation figure-of-merit (FoM) value of 1.9. In the case of LPF, the energy resolutions of 12.0% and 12.9% were achieved for the Ce:GAGG (top layer) and LYSO:Ce (bottom layer), respectively. CRTs (coincidence resolving times) of 314 ps and 263 ps were recorded for the top and bottom layers, respectively. The DOI-separation FoM value of 1.5 was achieved in this setup. Results show that the proposed method can provide excellent discrete DOI positioning accuracy without compromising the timing performance of the detector.

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