Brunner S E, Schaart D R
Radiation Science and Technology, Delft University of Technology, Mekelweg 15, 2629JB Delft, Netherlands.
Phys Med Biol. 2017 Jun 7;62(11):4421-4439. doi: 10.1088/1361-6560/aa6a49. Epub 2017 Mar 30.
Due to detector developments in the last decade, the time-of-flight (TOF) method is now commonly used to improve the quality of positron emission tomography (PET) images. Clinical TOF-PET systems based on L(Y)SO:Ce crystals and silicon photomultipliers (SiPMs) with coincidence resolving times (CRT) between 325 ps and 400 ps FWHM have recently been developed. Before the introduction of L(Y)SO:Ce, BGO was used in many PET systems. In addition to a lower price, BGO offers a superior attenuation coefficient and a higher photoelectric fraction than L(Y)SO:Ce. However, BGO is generally considered an inferior TOF-PET scintillator. In recent years, TOF-PET detectors based on the Cherenkov effect have been proposed. However, the low Cherenkov photon yield in the order of ∼10 photons per event complicates energy discrimination-a severe disadvantage in clinical PET. The optical characteristics of BGO, in particular its high transparency down to 310 nm and its high refractive index of ∼2.15, are expected to make it a good Cherenkov radiator. Here, we study the feasibility of combining event timing based on Cherenkov emission with energy discrimination based on scintillation in BGO, as a potential approach towards a cost-effective TOF-PET detector. Rise time measurements were performed using a time-correlated single photon counting (TCSPC) setup implemented on a digital photon counter (DPC) array, revealing a prompt luminescent component likely to be due to Cherenkov emission. Coincidence timing measurements were performed using BGO crystals with a cross-section of 3 mm × 3 mm and five different lengths between 3 mm and 20 mm, coupled to DPC arrays. Non-Gaussian coincidence spectra with a FWHM of 200 ps were obtained with the 27 mm BGO cubes, while FWHM values as good as 330 ps were achieved with the 20 mm long crystals. The FWHM value was found to improve with decreasing temperature, while the FWTM value showed the opposite trend.
由于过去十年中探测器技术的发展，飞行时间（TOF）方法现在常用于提高正电子发射断层扫描（PET）图像的质量。最近已开发出基于L（Y）SO:Ce晶体和硅光电倍增管（SiPM）的临床TOF-PET系统，其符合分辨时间（CRT）在325 ps至400 ps FWHM之间。在引入L（Y）SO:Ce之前，许多PET系统使用的是BGO。除了价格较低外，BGO的衰减系数和光电分数均优于L（Y）SO:Ce。然而，BGO通常被认为是一种较差的TOF-PET闪烁体。近年来，基于切伦科夫效应的TOF-PET探测器已被提出。然而，每个事件中切伦科夫光子产额低至约10个光子，这使得能量分辨变得复杂——这在临床PET中是一个严重的缺点。BGO的光学特性，特别是其在310 nm以下的高透明度和高达约2.15的高折射率，有望使其成为一种良好的切伦科夫辐射体。在此，我们研究了将基于切伦科夫发射的事件计时与基于BGO闪烁的能量分辨相结合的可行性，这是一种潜在的实现经济高效的TOF-PET探测器的方法。使用在数字光子计数器（DPC）阵列上实现的时间相关单光子计数（TCSPC）装置进行了上升时间测量，结果显示出一个可能归因于切伦科夫发射的快速发光成分。使用横截面为3 mm×3 mm且长度在3 mm至20 mm之间的五种不同长度的BGO晶体与DPC阵列耦合进行了符合计时测量。27 mm的BGO立方体获得了半高宽为200 ps的非高斯符合谱，而20 mm长的晶体实现了高达330 ps的半高宽值。发现半高宽值随温度降低而改善，而半高全宽值则呈现相反趋势。
