The National Electrical Manufacturers Association’s (NEMA) NU 4-2008 standard specifies methodology for evaluating the performance of small-animal PET scanners. The standard’s goal is to enable comparison of different PET scanners over a wide range of technologies and geometries used. In this work, we discuss if the NEMA standard meets these goals and we point out potential flaws and improvements to the standard.For the evaluation of spatial resolution, the NEMA standard mandates the use of filtered backprojection reconstruction. This reconstruction method can introduce star-like artifacts for detectors with an anisotropic spatial resolution, usually caused by parallax error. These artifacts can then cause a strong dependence of the resulting spatial resolution on the size of the projection window in image space, whose size is not fully specified in the NEMA standard. If the PET ring has detectors which are perpendicular to a Cartesian axis, then the resolution along this axis will typically improve with larger projection windows.We show that the standard’s equations for the estimation of the random rate for PET systems with intrinsic radioactivity are circular and not satisfiable. However, a modified version can still be used to determine an approximation of the random rates under the assumption of negligible random rates for small activities and a constant scatter fraction. We compare the resulting estimated random rates to random rates obtained using a delayed coincidence window and two methods based on the singles rates. While these methods give similar estimates, the estimation method based on the NEMA equations overestimates the random rates.In the NEMA standard’s protocol for the evaluation of the sensitivity, the standard specifies to axially step a point source through the scanner and to take a different scan for each source position. Later, in the data analysis section, the standard does not specify clearly how the different scans have to be incorporated into the analysis, which can lead to unclear interpretations of publicized results.The standard’s definition of the recovery coefficients in the image quality phantom includes the maximum activity in a region of interest, which causes a positive correlation of noise and recovery coefficients. This leads to an unintended trade-off between desired uniformity, which is negatively correlated with variance (i.e., noise), and recovery.With this work, we want to start a discussion on possible improvements in a next version of the NEMA NU-4 standard.

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关于小动物正电子发射断层扫描仪性能测量的NEMA NU 4-2008标准的评论

美国国家电气制造商协会（NEMA）NU 4-2008标准指定了评估小动物PET扫描仪性能的方法。该标准的目标是使所使用的各种技术和几何形状的不同PET扫描仪能够进行比较。在这项工作中，我们讨论了NEMA标准是否满足这些目标，并指出了该标准的潜在缺陷和改进。对于空间分辨率的评估，NEMA标准要求使用滤波后投影重建。这种重建方法可以为通常由视差误差引起的具有各向异性空间分辨率的探测器引入星形伪影。这些假象会导致结果空间分辨率强烈依赖于图像空间中投影窗口的大小，其尺寸未在NEMA标准中完全指定。如果PET环具有垂直于笛卡尔轴的检测器，则沿该轴的分辨率通常会随着较大的投影窗口而提高。我们证明，用于估计具有固有放射性的PET系统的随机率的标准方程为圆形且不能令人满意。但是，在小活动和恒定散射分数可忽略不计的随机率的假设下，修改后的版本仍可用于确定随机率的近似值。我们将所得的估计随机率与使用延迟重合窗口和基于单率的两种方法获得的随机率进行比较。尽管这些方法给出了相似的估计，但是基于NEMA方程的估计方法却高估了随机率。在NEMA标准的灵敏度评估协议中，该标准规定轴向点光源穿过扫描仪，并对每个光源位置进行不同的扫描。后来，在数据分析部分中，该标准没有明确规定必须将不同扫描纳入分析的方式，这可能导致对公开结果的理解不清楚。该标准对图像质量体模中恢复系数的定义包括：感兴趣区域中的最大活动，这会导致噪声和恢复系数呈正相关。这导致在期望的均匀度（与差异（即噪声）和恢复负相关）之间进行了意想不到的折衷。通过这项工作，