Washington University School of Medicine, St. Louis, MO 63110, USA.
Med Phys. 2012 Feb;39(2):1029-41. doi: 10.1118/1.3678489.
Iterative reconstruction algorithms are becoming more commonly employed in positron emission tomography (PET) imaging; however, the quantitative accuracy of the reconstructed images still requires validation for various levels of contrast and counting statistics.
The authors present an evaluation of the quantitative accuracy of the 3D maximum a posteriori (3D-MAP) image reconstruction algorithm for dynamic PET imaging with comparisons to two of the most widely used reconstruction algorithms: the 2D filtered-backprojection (2D-FBP) and 2D-ordered subsets expectation maximization (2D-OSEM) on the Siemens microPET scanners. The study was performed for various levels of count density encountered in typical dynamic scanning as well as the imaging of cardiac activity concentration in small animal studies on the Focus 120. Specially designed phantoms were used for evaluation of the spatial resolution, image quality, and quantitative accuracy. A normal mouse was employed to evaluate the accuracy of the blood time activity concentration extracted from left ventricle regions of interest (ROIs) within the images as compared to the actual blood activity concentration measured from arterial blood sampling.
For MAP reconstructions, the spatial resolution and contrast have been found to reach a stable value after 20 iterations independent of the β values (i.e., hyper parameter which controls the weight of the penalty term) and count density within the frame. The spatial resolution obtained with 3D-MAP reaches values of ∼1.0 mm with a β of 0.01 while the 2D-FBP has value of 1.8 mm and 2D-OSEM has a value of 1.6 mm. It has been observed that the lower the hyper parameter β used in MAP, more iterations are needed to reach the stable noise level (i.e., image roughness). The spatial resolution is improved by using a lower β value at the expense of higher image noise. However, with similar noise level the spatial resolution achieved by 3D-MAP was observed to be better than that by 2D-FBP or 2D-OSEM. Using an image quality phantom containing hot spheres, the estimated activity concentration in the largest sphere has the expected concentration relative to the background area for all the MAP images. The obtained recovery coefficients have been also shown to be almost independent of the count density. 2D-FBP and 2D-OSEM do not perform as well, yielding recovery coefficients lower than those observed with 3D-MAP (approximately 33% lower for the smallest sphere). However, a small positive bias was observed in MAP reconstructed images for frames of very low count density. This bias is present in the uniform area for count density of less than 0.05 × 10(6) counts/ml. For the dynamic mouse study, it was observed that 3D-MAP (even gated at diastole) cannot predict accurately the blood activity concentration due to residual spill-over activity from the myocardium into the left ventricle (approximately 15%). However, 3D-MAP predicts blood activity concentration closer to blood sampling than 2D-FBP.
The authors observed that 3D-MAP produces more accurate activity concentration estimates than 2D-FBP or 2D-OSEM at all practical levels of statistics and contrasts due to improved spatial resolution leading to lesser partial volume effect.
迭代重建算法在正电子发射断层扫描（PET）成像中越来越普遍地使用；然而，为了各种对比度和计数统计水平，仍然需要对重建图像的定量准确性进行验证。
作者提出了对 3D 最大后验（3D-MAP）图像重建算法在动态 PET 成像中的定量准确性的评估，该算法与最广泛使用的两种重建算法进行了比较：2D 滤波反投影（2D-FBP）和 2D 有序子集期望最大化（2D-OSEM），使用西门子 microPET 扫描仪进行比较。研究了在典型动态扫描中遇到的各种计数密度水平，以及在小动物研究中使用 Focus 120 对心脏活动浓度进行成像。专门设计的体模用于评估空间分辨率、图像质量和定量准确性。使用正常小鼠评估从图像的左心室 ROI 中提取的血液时间活动浓度与从动脉血样测量的实际血液活动浓度之间的准确性。
对于 MAP 重建，已经发现空间分辨率和对比度在框架内的β值（即控制惩罚项权重的超参数）和计数密度达到稳定值后达到稳定值，与β值（即控制惩罚项权重的超参数）无关。使用β值为 0.01 的 3D-MAP 获得的空间分辨率达到约 1.0mm，而 2D-FBP 的空间分辨率为 1.8mm，2D-OSEM 的空间分辨率为 1.6mm。已经观察到，在 MAP 中使用的超参数β越低，达到稳定噪声水平（即图像粗糙度）所需的迭代次数就越多。使用较低的β值可以提高空间分辨率，但会增加图像噪声。然而，在具有相似噪声水平的情况下，观察到 3D-MAP 获得的空间分辨率优于 2D-FBP 或 2D-OSEM。使用包含热球的图像质量体模，在所有 MAP 图像中，最大球体内的估计活动浓度相对于背景区域具有预期的浓度。还发现获得的恢复系数几乎与计数密度无关。2D-FBP 和 2D-OSEM 的性能不如 3D-MAP，恢复系数低于 3D-MAP 观察到的恢复系数（最小球体内约低 33%）。然而，在非常低计数密度的帧中，MAP 重建图像中观察到一个小的正偏差。该偏差存在于计数密度小于 0.05×10(6) 计数/ml 的均匀区域中。对于动态小鼠研究，观察到 3D-MAP（即使在舒张期门控）由于心肌中的残留溢出活动进入左心室（约 15%），不能准确预测血液活动浓度。然而，3D-MAP 预测的血液活动浓度比 2D-FBP 更接近血液采样。
作者观察到，由于空间分辨率的提高导致的较小的部分容积效应，3D-MAP 在所有实际的统计和对比度水平上产生比 2D-FBP 或 2D-OSEM 更准确的活动浓度估计。
