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Molecular imaging with PET is a powerful technique used primarily for diagnosis, treatment selection, treatment monitoring, and research in cancer (1) and neuropsychiatric disorders (2). Despite its high molecular specificity, quantitative nature, and clinical availability, PET has not been able to achieve its full potential as the go-to molecular imaging modality, largely because of its relatively poor spatial resolution, currently on the order of 3−6 mm (3,4). With this kind of spatial resolution, it is not possible to measure target density in small nodules or in many human and rodent brain regions relevant to disease etiology and pathophysiology.
Depth-encoding PET detector modules have been developed to mitigate parallax error (mispositioning of the line of response) for long scintillator crystals (5). These modules enable small-diameter PET detector rings with a reduced component cost per ring, large solid-angle coverage for increased sensitivity, and a reduced contribution from annihilation γ-ray acollinearity on spatial resolution when using crystals with a small cross-sectional area (4,6). In addition, depth-of-interaction (DOI) information can be used to deconvolve optical photon transport in long crystals, thus improving timing resolution (7,8). Depth-encoding detectors based on dual-ended readout achieve the best continuous DOI resolution of less than 2 mm (9,10). High-resolution PET systems such as the mammography-dedicated Clear-PEM have been developed using dual-ended DOI readout detectors (11), but these systems are too costly to be commercialized because of the large number of readout electronics compared with standard single-ended-readout PET scanners. A recently developed high-resolution variant of these detectors shows relatively poor energy and timing resolutions because the crystal-readout interface includes the use of glass light-guides, which are required to achieve accurate crystal identification (12). Alternative single-ended-readout detector modules to obtain DOI information have been proposed, such as multilayer phoswich blocks (13,14), retroreflectors for modules with monolithic scintillators (15), and other custom reflector designs (16,17). However, in all these designs, tradeoffs exist among depth encoding, cost, scintillator-to-readout coupling ratio, crystal identification accuracy, energy resolution, and timing resolution. To mitigate these tradeoffs, an ideal depth-encoding detector module is one with single-ended readout in which the crystal array is coupled directly to silicon photomultiplier (SiPM) pixels, without any intermediate glass light-guide, to minimize sharing of downward-traveling scintillation photons (i.e., photons traveling toward the SiPM) across multiple pixels and retain a good timing resolution. In addition, upward-traveling photons (i.e., photons traveling toward the light-guide), which do not contribute to the timing information, should be redirected via 180° bending of their paths toward the nearest neighboring SiPMs to retain good energy and DOI resolution and mimic the behavior of dual-ended depth-encoding readout detectors.
Detector modules consisting of depolished multicrystal scintillator arrays coupled 4-to-1 to SiPM pixels on one side and a uniform glass light-guide on the opposite side (Fig. 1A) have recently been investigated in efforts to develop a practical and cost-effective high-resolution time-of-flight PET scanner, as well as achieve continuous DOI localization using single-ended readout (8,18,19). In these detector modules, an energy weighted-average method is used for crystal identification to achieve energy and DOI resolutions of 9% and 3 mm in full width at half maximum (FWHM), respectively, using 1.53 × 1.53 × 15 mm crystals and 3 × 3 mm SiPM pixels (8). However, these arrays suffer from poor crystal identification along their edges and corners due to the lack of light-sharing neighbors (19), an issue that must be addressed since the edge and corner pixels comprise 75% and 44% of 4 × 4 and 8 × 8 SiPM readout chips, respectively. Also, intercrystal light sharing is inefficient when using a uniform glass light-guide since many upward-traveling photons are reflected back into the primary column and the rest are isotropically shared with a gaussian intensity distribution among neighbors. The problem with isotropic light sharing is the distribution of low-intensity signal across many SiPMs (Fig. 1A), the integrity of which will be severely affected by dark counts, resulting in degraded energy and DOI resolutions.
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We introduce the concept of Prism-PET (20), a single-ended-readout depth-encoding detector module that uses a specialized pattern of segmented prismatoid light-guides (Fig. 1B). Our Prism-PET detector modules have several key features. First, we use 3 distinct prismatoid designs (center, edge, and corner prismatoids) to mitigate edge and corner artifacts and thus achieve uniform crystal identification performance. Second, we confine intercrystal light sharing to only the nearest SiPM neighbors to create a deterministic and anisotropic intercrystal light-sharing pattern and maximize the signal-to-background ratio on those SiPMs to improve both energy and DOI resolutions. The segmentation pattern is the key feature that improves crystal identification by decoupling adjacent crystals that would otherwise have similar readout patterns; thus, the shape of each prismatoid is interchangeable (cuboids, pyramids, wedges, prisms, cupolas, frusta, and others). In this study, right triangular prisms were used. Third, the right triangular prisms enhance intercrystal light-sharing ratios, thus improving both crystal identification and DOI resolution. When optical photons enter the hypotenuse of the right triangular prisms, they undergo 180° deviation, efficiently guiding them to neighboring crystals, which are coupled to different readout pixels because of the offset crystal-to-prism coupling scheme with respect to crystal-to-pixel coupling (Fig. 2). Using experimental measurements, we demonstrate the advantages of our design in terms of crystal identification, energy resolution, and DOI resolution, including how Prism-PET enables up to 9-to-1 crystal-to-readout coupling, which can be used to substantially improve spatial resolution without increasing the number of readout channels (Figs. 2C and 2D).
We have developed, fabricated, and characterized our proposed Prism-PET detector module, which is a true single-ended equivalent of a dual-ended depth-encoding readout using efficient 180° light-bending reflectors for enhanced light sharing. We achieved 2.5-mm-FWHM DOI resolution and up to 9-to-1 scintillator-to-SiPM coupling for high spatial resolution while directly coupling the crystal array to the SiPM pixels to minimize light leakage and retain high photon detection efficiency, which is required for good timing resolution. The top-side reflector comprises an optimized pattern of segmented prismatoid light-guides for efficient redirection of scintillation photon paths from the primary crystal to selected nearest-neighboring SiPMs, thus closely mimicking the operation of dual-ended-readout detectors. This creates an anisotropic and deterministic pattern of signal that can be used to decompose side-by-side Compton scattering events into their constituent energy and DOI information for the purpose of scatter recovery. Thus, we can expect to achieve high and uniform spatial resolution (9-to-1 coupling of ∼1-mm crystals; absence of edge and corner artifacts due to enhanced light sharing; reduced spatial blur due to Compton-scattered photons via scatter recovery), high sensitivity (20-mm-thick detectors and intercrystal Compton scatter recovery), and good energy and timing resolutions (especially after applying DOI correction) in compact systems (DOI encoding eliminates parallax error and permits smaller ring-diameter). With these unique combinations of features, cost-effective and compact time-of-flight DOI-Compton PET scanners based on Prism-PET modules could be developed for small-animal and human organ-specific functional and molecular imaging.
We gratefully acknowledge PETsys Electronics, SA, Portugal, for scientific discussions.
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