Hallen Patrick, Schug David, Schulz Volkmar
Department of Physics of Molecular Imaging Systems, Institute for Experimental Molecular Imaging, RWTH Aachen University, Pauwelstraße 19, Aachen, 52074, Germany.
Hyperion Hybrid Imaging Systems GmbH, Pauwelstraße 19, Aachen, 52074, Germany.
EJNMMI Phys. 2020 Feb 24;7(1):12. doi: 10.1186/s40658-020-0279-2.
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.
美国国家电气制造商协会（NEMA）的NU 4 - 2008标准规定了评估小动物正电子发射断层扫描（PET）扫描仪性能的方法。该标准的目标是能够在广泛使用的各种技术和几何结构下，对不同的PET扫描仪进行比较。在这项工作中，我们讨论了NEMA标准是否实现了这些目标，并指出了该标准潜在的缺陷和改进之处。
对于空间分辨率的评估，NEMA标准规定使用滤波反投影重建。这种重建方法对于具有各向异性空间分辨率的探测器可能会引入类似星状的伪影，通常是由视差误差引起的。这些伪影会导致所得空间分辨率强烈依赖于图像空间中投影窗口的大小，而NEMA标准并未完全明确该窗口的大小。如果PET环的探测器垂直于笛卡尔坐标轴，那么沿着该轴的分辨率通常会随着投影窗口变大而提高。
我们表明，该标准中用于估计具有固有放射性的PET系统随机率的方程是循环的且无法满足。然而，在小活度下随机率可忽略且散射分数恒定的假设下，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符合窗口以及基于单计数率的两种方法得到的随机率进行了比较。虽然这些方法给出的估计值相似，但基于NEMA方程的估计方法高估了随机率。
在NEMA标准的灵敏度评估协议中，标准规定将一个点源沿轴向步进通过扫描仪，并对每个源位置进行不同的扫描。后来，在数据分析部分，标准没有明确说明如何将不同的扫描纳入分析，这可能导致对公布结果的解释不清晰。
该标准在图像质量体模中对恢复系数的定义包括感兴趣区域内的最大活度，这导致噪声与恢复系数呈正相关。这在期望的均匀性（与方差即噪声呈负相关）和恢复之间导致了意外的权衡。
通过这项工作，我们希望开启关于NEMA NU - 4标准下一版本可能改进之处的讨论。
