Central Research Laboratory, Hamamatsu Photonics K. K., 5000, Hirakuchi, Hamakita-ku, Hamamatsu City, 434-8601, Japan.
School of Allied Health Sciences, Kitasato University, 1-15-1, Kitasato, Minami-ku, Sagamihara City, 252-0373, Japan.
Med Phys. 2018 May;45(5):1999-2008. doi: 10.1002/mp.12851. Epub 2018 Mar 23.
Cherenkov radiation has recently received attention due to its prompt emission phenomenon, which has the potential to improve the timing performance of radiation detectors dedicated to positron emission tomography (PET). In this study, a Cherenkov-based three-dimensional (3D) position-sensitive radiation detector was proposed, which is composed of a monolithic lead fluoride (PbF ) crystal and a photodetector array of which the signals can be readout independently.
Monte Carlo simulations were performed to estimate the performance of the proposed detector. The position- and time resolution were evaluated under various practical conditions. The radiator size and various properties of the photodetector, e.g., readout pitch and single photon timing resolution (SPTR), were parameterized. The single photon time response of the photodetector was assumed to be a single Gaussian for the simplification. The photo detection efficiency of the photodetector was ideally 100% for all wavelengths. Compton scattering was included in simulations, but partly analyzed. To estimate the position at which a γ-ray interacted in the Cherenkov radiator, the center-of-gravity (COG) method was employed. In addition, to estimate the depth-of-interaction (DOI) principal component analysis (PCA), which is a multivariate analysis method and has been used to identify the patterns in data, was employed. The time-space distribution of Cherenkov photons was quantified to perform PCA. To evaluate coincidence time resolution (CTR), the time difference of two independent γ-ray events was calculated. The detection time was defined as the first photon time after the SPTR of the photodetector was taken into account.
The position resolution on the photodetector plane could be estimated with high accuracy, by using a small number of Cherenkov photons. Moreover, PCA showed an ability to estimate the DOI. The position resolution heavily depends on the pitch of the photodetector array and the radiator thickness. If the readout pitch were ideally 0 and practically 3 mm, a full-width at half-maximum (FWHM) of 0.348 and 1.92 mm was achievable with a 10-mm-thick PbF crystal, respectively. Furthermore, first-order correlation could be observed between the primary principal component and the true DOI. To obtain a coincidence timing resolution better than 100-ps FWHM with a 20-mm-thick PbF crystal, a photodetector with SPTR of better than σ = 30 ps was necessary.
From these results, the improvement of SPTR allows us to achieve CTR better than 100-ps FWHM, even in the case where a 20-mm-thick radiator is used. Our proposed detector has the potential to estimate the 3D interaction position of γ-rays in the radiator, using only time and space information of Cherenkov photons.
由于切伦科夫辐射具有快速发射的现象，因此最近引起了人们的关注，这种现象有可能提高专门用于正电子发射断层扫描（PET）的辐射探测器的定时性能。在这项研究中，提出了一种基于切伦科夫的三维（3D）位置灵敏辐射探测器，它由单片氟化铅（PbF2）晶体和一个可以独立读取信号的光电探测器阵列组成。
通过蒙特卡罗模拟来估计所提出的探测器的性能。在各种实际条件下评估位置和时间分辨率。参数化了辐射器的尺寸和光电探测器的各种特性，例如，读出间距和单个光子定时分辨率（SPTR）。为了简化起见，假设光电探测器的单光子时间响应为单个高斯。对于所有波长，光电探测器的光探测效率理想地为 100％。在模拟中包括康普顿散射，但仅进行了部分分析。为了估计 γ射线在切伦科夫辐射器中相互作用的位置，采用了重心（COG）方法。此外，为了估计深度交互（DOI）主成分分析（PCA），采用了一种多元分析方法，并已用于识别数据中的模式。量化了切伦科夫光子的时空分布以进行 PCA。为了评估符合时间分辨率（CTR），计算了两个独立γ射线事件的时间差。检测时间定义为考虑光电探测器的 SPTR 后的第一个光子时间。
通过使用少量的切伦科夫光子，可以准确估计光电探测器平面上的位置分辨率。此外，PCA 显示出估计 DOI 的能力。位置分辨率严重依赖于光电探测器阵列的间距和辐射器的厚度。如果读出间距理想情况下为 0 且实际为 3mm，则使用 10mm 厚的 PbF2 晶体可分别实现 0.348 和 1.92mm 的半高全宽（FWHM）。此外，初级主成分与真实 DOI 之间可以观察到一阶相关性。为了获得具有 20mm 厚 PbF2 晶体的优于 100-ps FWHM 的符合时间分辨率，需要具有优于σ=30ps 的 SPTR 的光电探测器。
从这些结果可以看出，提高 SPTR 可以使我们即使在使用 20mm 厚的辐射器的情况下，也能实现优于 100-ps FWHM 的 CTR。我们提出的探测器具有仅使用切伦科夫光子的时空信息来估计辐射器中 γ 射线的 3D 相互作用位置的潜力。
