Thompson C J
McConnell Brain Imaging Center, Montreal Neurological Institute, Quebec, Canada.
J Cereb Blood Flow Metab. 1991 Mar;11(2):A31-7. doi: 10.1038/jcbfm.1991.34.
The rates for true coincident events, scattered events, and singles in a Positron Emission Tomograph (PET) depend on the collimator size and shape. Monte Carlo techniques were used to compare efficiencies for wanted and unwanted events in multislice PET (MS-PET) and open collimator positron volumetric imaging (PVI) configurations. All systems used cylindrical arrays of bismuth germanate (BGO) or NaI crystals 51 cm in diameter and 10 cm deep suitable for whole brain imaging. The PVI systems detect about five times more true coincident events at low activity concentrations, but their scatter fraction is about three times higher. They are also much more sensitive to activity outside the scan field. As well as causing random counts when they fell within the energy acceptance window, single events are the main cause of deadtime. When the detectors are made from light-encoded blocks deadtime is the major limitation at high count rates. When discrete crystals are used, the efficiency is lower and the random count rates are a more significant source of noise. Noise-effective count rates are used to compare the relative cost in system performance among different systems and sources of noise.
正电子发射断层扫描仪（PET）中真符合事件、散射事件和单光子事件的发生率取决于准直器的尺寸和形状。采用蒙特卡罗技术比较了多层PET（MS-PET）和开放式准直器正电子容积成像（PVI）配置中所需事件和不需要事件的效率。所有系统均使用直径51厘米、深度10厘米的锗酸铋（BGO）或碘化钠晶体圆柱形阵列，适用于全脑成像。PVI系统在低活度浓度下检测到的真符合事件大约多五倍，但其散射分数大约高三倍。它们对扫描视野外的活度也更为敏感。单光子事件不仅在落入能量接受窗时会产生随机计数，而且是造成死时间的主要原因。当探测器由光编码模块制成时，死时间是高计数率下的主要限制因素。当使用离散晶体时，效率较低，随机计数率是更显著的噪声源。噪声有效计数率用于比较不同系统和噪声源之间系统性能的相对成本。
