School of Allied Health Sciences, Kitasato University, 1-15-1, Kitasato, Minamiku, Sagamihara, Kanagawa, 252-0373, Japan.
Phys Med Biol. 2011 Sep 21;56(18):6031-45. doi: 10.1088/0031-9155/56/18/016. Epub 2011 Aug 24.
The uncertainty of radioactivity concentrations measured with positron emission tomography (PET) scanners ultimately depends on the uncertainty of the calibration factors. A new practical calibration scheme using point-like (22)Na radioactive sources has been developed. The purpose of this study is to theoretically investigate the effects of the associated 1.275 MeV γ rays on the calibration factors. The physical processes affecting the coincidence data were categorized in order to derive approximate semi-quantitative formulae. Assuming the design parameters of some typical commercial PET scanners, the effects of the γ rays as relative deviations in the calibration factors were evaluated by semi-quantitative formulae and a Monte Carlo simulation. The relative deviations in the calibration factors were less than 4%, depending on the details of the PET scanners. The event losses due to rejecting multiple coincidence events of scattered γ rays had the strongest effect. The results from the semi-quantitative formulae and the Monte Carlo simulation were consistent and were useful in understanding the underlying mechanisms. The deviations are considered small enough to correct on the basis of precise Monte Carlo simulation. This study thus offers an important theoretical basis for the validity of the calibration method using point-like (22)Na radioactive sources.
正电子发射断层扫描（PET）扫描仪测量放射性浓度的不确定性最终取决于校准因子的不确定性。已经开发了一种使用点状（22）Na 放射性源的新实用校准方案。本研究的目的是从理论上研究相关的 1.275 MeV γ 射线对校准因子的影响。为了推导出近似的半定量公式，对影响符合数据的物理过程进行了分类。假设一些典型商业 PET 扫描仪的设计参数，通过半定量公式和蒙特卡罗模拟评估了γ射线对校准因子的相对偏差。校准因子的相对偏差小于 4％，具体取决于 PET 扫描仪的细节。由于拒绝散射γ射线的多次符合事件而导致的事件损失具有最强的影响。半定量公式和蒙特卡罗模拟的结果是一致的，有助于理解潜在的机制。这些偏差被认为足够小，可以根据精确的蒙特卡罗模拟进行校正。因此，这项研究为使用点状（22）Na 放射性源的校准方法的有效性提供了重要的理论基础。
