Characterization of stacked-crystal PET detector designs for measurement of both TOF and DOI

Abstract

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 (LaBr3), with various Cerium dopant concentrations, including pure Cerium Bromide (CeBr3). 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 LaBr3 and CeBr3. 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 mm2 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 LaBr3(Ce:20%) crystal on top of a 15-mm long CeBr3 crystal, readout using a PMT. A DOI accuracy of 95% was measured with a stack of two, identical, 12-mm long, CeBr3 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, CeBr3 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 LaBr3/CeBr3 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 CeBr3 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.

Time-over-threshold for pulse shape discrimination in a time-of-flight phoswich PET detector

“…The position of the layer-deconvolved photopeaks correspond well with the ones observed in the side-on energy spectra, verifying the layer separation achieved with ToT. The energy resolution without correction for SiPM non-linearity (9.7 % for LYSO and 11.3 % for LSO) achieved with the phoswich detector are comparable to recent studies also using lutetium-based crystals (Schmall et al 2015, Cates & Levin 2016). …”

“…It has been shown in a recent study using stacked crystal design that SiPMs outperformed PMTs in coincidence time resolution (Schmall et al 2015). SiPMs are also compact in size compared to PMTs, which allows one-to-one coupling between the crystal elements and SiPM pixels.…”

Delay grid multiplexing: simple time-based multiplexing and readout method for silicon photomultipliers

“…Subsequently, the precise corner node arrival times obtained at the steepest slope of the signal could improve the flood-map quality. Furthermore, the depth-of-interaction measurement that involves pulse shape discrimination (Du et al 2009, Schmall et al 2015 and pulse pile-up recovery (Haselman et al 2012) can be applied after multiplexing thanks to minimal signal distortion by the multiplexing circuit. However, a DRS4 serves as the time stretcher, and thus requires the external ADC and FPGA that can complicate the readout circuit (Ritt et al 2010).…”

A multiplexed TOF and DOI capable PET detector using a binary position sensitive network

“…Many designs have an increased number of photosensors such as dual-ended readout (Kang, Ko, Rhee, Kim, Lee & Hong 2015), or multi-layered detectors (Yeom, Vinke & Levin 2014). Other methods do not use increased number of photodetectors such as using scintillation shape (rise or decay times) (Schmall, Surti & Karp 2015) (Roncali, Schmall, Viswanath, Berg & Cherry 2014) or light sharing (Van Dam, Borghi, Seifert & Schaart 2013) (Ito, Lee & Lee 2013) to give DOI information. However, many of these methods require active cooling or a significant increase in PET detector readout complexity to measure the light spread or scintillation shape accurately.…”