Timing-performance evaluation of Cherenkov-based radiation detectors

https://doi.org/10.1016/j.nima.2019.01.034Get rights and content

Abstract

With the upgradation of detector components, such as scintillators and photodetectors, the PET-image signal-to-noise ratio of time-of-flight positron emission tomography (TOF-PET) systems has been improved, compared to those of ordinary nonTOF-PET systems. A TOF-PET with an ultrahigh time resolution, for example a coincidence time resolution (CTR) better than few tens of picoseconds, can not only improve the image quality, but also remove the image reconstruction process, significantly impacting medical imaging. Therefore, it is crucial to develop a high-time resolution PET detector. We focus on the prompt emission of Cherenkov radiation, owing to the instantaneousness of which, a high time resolution can be expected. One of the candidates for the Cherenkov radiator is lead fluoride (PbF2) due it has excellent properties, including transparency toward the ultraviolet region, high refractive index (n = 1.82), and high density (7.77 g/cm3). Moreover, it does not contain radioisotopes, unlike lutetium-based scintillators, which are commonly used in the currently available TOF-PET detectors. In this work, we experimentally investigate the timing performance of PbF2-based Cherenkov detectors, breaking down the timing performance into physical components. 3×3×5 mm3 and 9.6×9.6×5 mm3 PbF2 crystals are used as Cherenkov radiators; both are attached to a microchannel plate photomultiplier tube (MCP-PMT) because the single channel MCP-PMT is one of the best photodetectors in terms of the SPTR, which is 25 ps full width at half maximum (FWHM). All the surfaces, except the end surface where the MCP-PMT is connected, are wrapped in black tape to suppress the reflections of the Cherenkov photons in the crystal. The CTR is measured by placing a detector pair face-to-face, using an 22Na point source, and an oscilloscope at 20 GS/s with a set bandwidth of 4.2 GHz. A CTR of 46.9 ps FWHM, corresponding to a position resolution of 7.0 mm, is obtained, consistent with our simulation results.

Introduction

There is a requirement for radiation detectors with high timing performances in time-of-flight positron emission tomography (TOF-PET) applications. The advancements in detector timing resolution have improved the image quality of PET systems, in terms of the signal-to-noise ratio (SNR). The contribution of the timing resolution is described by the following equation: SNRTOF-PETSNRnonTOF-PET=2D(cΔT),where D is the effective diameter of an object, c is the speed of light, and ΔT is the coincidence time resolution (CTR) of a PET detector [1]. If a CTR of 30 ps full width at half maximum (FWHM) is achieved, the position resolution along the line of response (LOR) is equivalent to 4.5 mm FWHM. As this position resolution is of the same order of the spatial (x, y) resolution of recent clinical TOF-PET systems [2], the direct determination of the annihilation point along the LOR, in clinical TOF-PET systems, is achievable [3]. Direct localization enables the removal of artifacts caused by image reconstruction processes, significantly impacting medical imaging. Therefore, it is crucial to pursue ultrahigh timing resolution down to 30 ps FWHM.

The timing resolution of scintillation-based detectors has been studied theoretically and experimentally for several decades [4], [5], [6]. Various research groups have already obtained CTRs better than 100 ps FWHM, using scintillation-based radiation detectors [7], [8]. These research has established that prompt emissions, which occur before the scintillation luminescence, play an important role in improving the timing resolution [9], [10], [11]. One of the prompt emissions that can be detected by photodetectors, such as photomultiplier tubes (PMTs) and silicon photomultipliers (SiPMs), is the Cherenkov radiation. In addition to prompt emission, single photon timing resolution (SPTR) also plays an important role [3], [12].

The timing performance of a Cherenkov-based detector for PET has been reported, using a fast photodetector [13]. The CTRs of a pair of detectors, composed of a lead fluoride (PbF2) crystal and a multianode microchannel plate PMT (MA-MCP-PMT, Hamamatsu Photonics K. K.), with an SPTR of σ=27 ps, were 71 and 95 ps FWHM using 5- and 15-mm thick radiators, respectively [13]. These results correspond to the simulation results of ref. [12], according to which, the CTR can be improved, if the SPTR is better than that of the MA-MCP-PMT. However, it has not been experimentally proved that a better SPTR definitely provides a better CTR.

In this paper, we present the experimentally obtained CTRs, using detectors composed of a PbF2 crystal and single channel MCP-PMT. A single channel MCP-PMT is used because it is one of the photodetectors with an SPTR better than that of the MA-MCP-PMT. The potential capability of a single-channel MCP-PMT as a fast timing detector is demonstrated.

Section snippets

Detector

The MCP-PMT (R3809, Hamamatsu Photonics K. K.) was used as the photodetector because it has the highest timing performance against a single photon; the SPTR is 25 ps FWHM [14], which is better by more than twice, compared to that of the MA-MCP-PMT used in ref. [13].

The number of emissions of Cherenkov photons within the Cherenkov photon wavelength interval between λ1 and λ2 (λ1<λ2) is theoretically described by the following equation: N=2παL(1λ11λ2)(1(1nβ)2),where N is the number of emitted

Results and discussion

Fig. 4-(a) illustrates the time difference between the signals from the two detectors, obtained using the experimental setup shown in Fig. 2. The broken line depicts the fitting result; the CTR is 46.9 ps FWHM, which corresponds to a position resolution of 7.0 mm. Thus, a position resolution better than 10 mm was achieved using the existing detectors. The two small peaks around the center peak were caused by the direct interaction of an annihilation γ-ray with the MCP. This ultrahigh timing

Conclusion

In this work, excellent timing performance was obtained using a Cherenkov-based detector, owing to its prompt emission and good SPTR. It was established that a CTR of 46.9 ps, corresponding to a position resolution of 7.0 mm, for annihilation γ-rays, is possible using the existing detectors. Thus, further improvement of the photodetector, in terms of the SPTR in particular, can definitely improve the timing performance and render the reconstruction process in TOF-PET systems unnecessary.

Acknowledgments

The authors thank Tomohide Omura and Ryoko Yamada of the Central Research Laboratory of Hamamatsu Photonics for their assistance and significant discussions.

References (16)

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