Trigila Carlotta, Roncali Emilie
Department of Biomedical Engineering, University of California, Davis, Davis, CA, United States.
Department of Radiology, University of California, Davis, Davis, CA, United States.
Front Phys. 2022;10. doi: 10.3389/fphy.2022.1028293. Epub 2022 Oct 6.
Using Cerenkov photons to improve detector timing resolution in time-of-flight positron emission tomography scanners is promising since they constitute most of the signal rising edge. The main challenge in using Cerenkov light is its low yield per photoelectric interaction, which requires optimizing their complex optical transport in the detector. Monte Carlo simulations unlock information unavailable through benchtop measurements and help better understand the Cerenkov photon behavior. Although the first Cerenkov photons are emitted forward, part of the early triggering signal is lost due to poor light extraction at the crystal-photodetector interface. In addition, the electron path in the crystal, that determines the Cerenkov photon direction, is tortuous due to multiple scattering, causing the Cerenkov photons emitted after a few scatters to no longer be forward-directed. In this context, the transit time spread in the crystal, highly dependent on the detector geometry, plays a crucial role in the photon detection time. In this work, we performed optical simulations in bismuth germanium oxide using 511 keV gamma with GATE to investigate the optical photons extraction when modifying the index of refraction at the crystal-photodetector interface and the crystal aspect ratio. The mean detection time of the first, second, and third detected optical and Cerenkov photon separately was studied as a function of the total number of Cerenkov detected per event. For each configuration, we calculated the expected mean detection time using the probability of detection. Thinner crystals led to lower expected detection times due to the reduced transit time in the crystal. Reducing the refractive index discontinuity at the crystal-photodetector interface decreased all configurations expected mean detection time values. We showed that it not only improves the optical photons (scintillation and Cerenkov) detection efficiency at the photodetector face but directly ameliorates the probability of detecting the fastest one, reducing the effect of thicker materials and of losing the first detected photon information, both crucial to reduce the detector timing resolution. Thanks to their prompt emission and directionality at emission, Cerenkov photons represent the first detected optical photon in most configurations but increasing their detection efficiency is crucial to detect the fastest one.
利用切伦科夫光子来提高飞行时间正电子发射断层扫描仪中的探测器时间分辨率是很有前景的，因为它们构成了大部分信号上升沿。使用切伦科夫光的主要挑战在于其每次光电相互作用的产额较低，这就需要优化其在探测器中的复杂光传输。蒙特卡罗模拟能够揭示通过台式测量无法获得的信息，并有助于更好地理解切伦科夫光子的行为。尽管最初的切伦科夫光子是向前发射的，但由于晶体 - 光电探测器界面处的光提取不佳，部分早期触发信号会丢失。此外，晶体中的电子路径（它决定了切伦科夫光子的方向）由于多次散射而曲折，导致经过几次散射后发射的切伦科夫光子不再向前传播。在这种情况下，晶体中的渡越时间展宽高度依赖于探测器的几何形状，在光子探测时间中起着至关重要的作用。在这项工作中，我们使用GATE在铋锗氧化物中对511 keV伽马射线进行了光学模拟，以研究在改变晶体 - 光电探测器界面处的折射率和晶体纵横比时的光学光子提取情况。分别研究了首次、第二次和第三次检测到的光学光子和切伦科夫光子的平均探测时间与每个事件中检测到的切伦科夫光子总数的函数关系。对于每种配置，我们使用检测概率计算了预期平均探测时间。更薄的晶体由于晶体中渡越时间的减少而导致预期探测时间更低。减小晶体 - 光电探测器界面处的折射率不连续性降低了所有配置的预期平均探测时间值。我们表明，这不仅提高了光电探测器表面的光学光子（闪烁光子和切伦科夫光子）探测效率，而且直接改善了检测最快光子的概率，减少了较厚材料的影响以及丢失首次检测到的光子信息的影响，这两者对于降低探测器时间分辨率都至关重要。由于切伦科夫光子在发射时具有即时发射和方向性，在大多数配置中它们是首次检测到的光学光子，但提高它们的探测效率对于检测最快的光子至关重要。
