Jeffrey P. Schmall

There is a growing interest in the design and construction of a PET scanner with a very long axial extent. One critical design challenge is the impact of the long axial extent on the scanner spatial resolution properties. In this work, we characterize the effect of parallax error in PET system designs having an axial field-of-view (FOV) of 198 cm (total-body PET scanner) using fully-3D Monte Carlo simulations. Two different scintillation materials were studied: LSO and LaBr ₃ . The crystal size in both cases was 4 × 4 × 20 mm ³ . Several different depth-of-interaction (DOI) encoding techniques were investigated to characterize the improvement in spatial resolution when using a DOI capable detector. To measure spatial resolution we simulated point sources in a warm background in the center of the imaging FOV, where the effects of axial parallax are largest, and at several positions radially offset from the center. Using a line-of-response based ordered-subset expectation maximization reconstruction algorithm we found that the axial resolution in an LSO scanner degrades from 4.8 mm to 5.7 mm (full width at half max) at the center of the imaging FOV when extending the axial acceptance angle ( α ) from ±12° (corresponding to an axial FOV of 18 cm) to the maximum of ±67°—a similar result was obtained with LaBr ₃ , in which the axial resolution degraded from 5.3 mm to 6.1 mm. For comparison we also measured the degradation due to radial parallax error in the transverse imaging FOV; the transverse resolution, averaging radial and tangential directions, of an LSO scanner was degraded from 4.9 mm to 7.7 mm, for a measurement at the center of the scanner compared to a measurement with a radial offset of 23 cm. Simulations of a DOI detector design improved the spatial resolution in all dimensions. The axial resolution in the LSO-based scanner, with α = ± 67°, was improved from 5.7 mm to 5.0 mm by incorporating a two-layer DOI detector. These results characterize the maximum axial blurring for a fully open 2 m long PET scanner and demonstrate that large sensitivity gains are possible with a modest reduction in resolution when using current clinical detector technology with no DOI capability.

A PET detector with good timing resolution and two-level depth-of-interaction (DOI) discrimination can be constructed using a single-ended readout of scintillator stacks of Lanthanum Bromide (LaBr ₃ ), with various Cerium dopant concentrations, including pure Cerium Bromide (CeBr ₃ ). The stacked crystal geometry creates a unique signal shape for interactions occurring in each layer, which can be used to identify the DOI, while retaining the inherently good timing properties of LaBr ₃ and CeBr ₃ . In this work, single pixel elements are used to optimize the choice of scintillator, coupling of layers, and type of photodetector, evaluating the performance using a fast, single-channel photomultiplier tube (PMT) and a single 4 × 4 mm ² silicon photomultiplier (SiPM). We also introduce a method to quantify and evaluate the DOI discrimination accuracy. From signal shape measurements using fast waveform sampling, we found that in addition to differences in signal rise times, between crystal layers, there were also differences in the signal fall times. A DOI accuracy of 98% was achieved using our classification method for a stacked crystal pair, consisting of a 15 mm long LaBr ₃ (Ce:20%) crystal on top of a 15 mm long CeBr ₃ crystal, readout using a PMT. A DOI accuracy of 95% was measured with a stack of two, identical, 12 mm long, CeBr ₃ crystals. The DOI accuracy of this crystal pair was reduced to 91% when using a SiPM for readout. For the stack of two, 12 mm long, CeBr ₃ crystals, a coincidence timing resolution (average of timing results from the top and bottom layer) of 199 ps was measured using a PMT, and this was improved to 153 ps when using a SiPM. These results show that with stacked LaBr ₃ /CeBr ₃ scintillators and fast waveform sampling nearly perfect DOI accuracy can be achieved with excellent timing resolution—timing resolution that is only minimally degraded compared to results from a single CeBr ₃ crystal of comparable length to the stacked crystals. The interface in the stacked crystal geometry itself plays a major role in creating the differences in signal shape and this can be used to construct stacked DOI detectors using the same scintillator type, thereby simplifying and broadening the application of this technique.

The high light output and fast decay time of LaBr ₃ :Ce scintillation detectors leads to excellent timing performance. To realize the potential of timing resolution with LaBr ₃ :Ce we have investigated the performance with SiPMs, which enable 1-to-1 coupling to individual crystals, and which have been optimized for the near-ultraviolet (NUV) scintillation light emission of LaBr ₃ :Ce. Coincidence timing resolution (CTR) of 100 ps was measured for a 4 ×4 ×5 mm ³ LaBr ₃ :30%Ce crystal directly coupled to a large-area 4 ×4 mm ² NUV-SiPM. Results show very little dependence on temperature, in the range of - 20 ^° to 20 ^° C, and bias voltage, from 2 V to 5 V over breakdown. Optimal performance was achieved at an over-voltage (OV) range of 3 V - 5 V, at which high gain and high photon detection efficiency are achieved. Though saturation was evident at 511 keV, an energy resolution of 6.8% was measured after correcting for non-linearity. We also measured a CTR of 110 ps for a 4 ×4 ×5 mm ³ LaBr3:5%Ce crystal and 245 ps for a 4 ×4 ×30 mm ³ LaBr ₃ :5%Ce crystal using the NUV-SIPM. The poorer timing measurement for the 30-mm long crystal is due mainly to a systematic shift in the time pick-off as a function of the depth-of-interaction. The excellent temperature stability, fast rise time, high gain, and low noise of the NUV-SiPM make it a practical and highly appealing photodetector for the readout of a LaBr ₃ :Ce TOF-PET detector.