College of Optical Sciences, University of Arizona, 1630 E. University Ave, Tucson, AZ, USA.
Department of Radiology and Radiological Sciences, Institute of Imaging Science, Vanderbilt University Medical Center, 1161 21st Avenue South, Nashville, TN, USA.
Med Phys. 2018 Jul;45(7):2952-2963. doi: 10.1002/mp.12952. Epub 2018 Jun 1.
In traditional multipinhole SPECT systems, image multiplexing - the overlapping of pinhole projection images - may occur on the detector, which can inhibit quality image reconstructions due to photon-origin uncertainty. One proposed system to mitigate the effects of multiplexing is the synthetic-collimator SPECT system. In this system, two detectors, a silicon detector and a germanium detector, are placed at different distances behind the multipinhole aperture, allowing for image detection to occur at different magnifications and photon energies, resulting in higher overall sensitivity while maintaining high resolution. The unwanted effects of multiplexing are reduced by utilizing the additional data collected from the front silicon detector. However, determining optimal system configurations for a given imaging task requires efficient parsing of the complex parameter space, to understand how pinhole spacings and the two detector distances influence system performance.
In our simulation studies, we use the ensemble mean-squared error of the Wiener estimator (EMSE ) as the figure of merit to determine optimum system parameters for the task of estimating the uptake of an I-labeled radiotracer in three different regions of a computer-generated mouse brain phantom. The segmented phantom map is constructed by using data from the MRM NeAt database and allows for the reduction in dimensionality of the system matrix which improves the computational efficiency of scanning the system's parameter space. To contextualize our results, the Wiener estimator is also compared against a region of interest estimator using maximum-likelihood reconstructed data.
Our results show that the synthetic-collimator SPECT system outperforms traditional multipinhole SPECT systems in this estimation task. We also find that image multiplexing plays an important role in the system design of the synthetic-collimator SPECT system, with optimal germanium detector distances occurring at maxima in the derivative of the percent multiplexing function. Furthermore, we report that improved task performance can be achieved by using an adaptive system design in which the germanium detector distance may vary with projection angle. Finally, in our comparative study, we find that the Wiener estimator outperforms the conventional region of interest estimator.
Our work demonstrates how this optimization method has the potential to quickly and efficiently explore vast parameter spaces, providing insight into the behavior of competing factors, which are otherwise very difficult to calculate and study using other existing means.
在传统的多孔径单光子发射计算机断层成像（SPECT）系统中，探测器上可能会发生针孔投影图像的重叠，即图像复用，这可能会由于光子起源的不确定性而抑制高质量的图像重建。一种减轻复用影响的系统是合成准直器 SPECT 系统。在该系统中，两个探测器，即硅探测器和锗探测器，放置在多针孔孔径的不同距离后面，允许在不同的放大率和光子能量下进行图像检测，从而在保持高分辨率的同时提高整体灵敏度。通过利用来自前置硅探测器的额外数据，可以减少复用的不良影响。然而，为给定的成像任务确定最佳系统配置需要有效地解析复杂的参数空间，以了解针孔间距和两个探测器距离如何影响系统性能。
在我们的模拟研究中，我们使用 Wiener 估计器的均方误差（EMSE）作为衡量标准，来确定用于估计计算机生成的老鼠脑模型中三个不同区域的 I 标记放射性示踪剂摄取的任务的最佳系统参数。分割后的幻影地图是通过使用 MRM NeAt 数据库的数据构建的，这允许降低系统矩阵的维数，从而提高扫描系统参数空间的计算效率。为了使我们的结果具有上下文相关性，Wiener 估计器也与使用最大似然重建数据的感兴趣区域估计器进行了比较。
我们的结果表明，在这个估计任务中，合成准直器 SPECT 系统比传统的多孔径 SPECT 系统表现更好。我们还发现，图像复用在合成准直器 SPECT 系统的系统设计中起着重要作用，最佳的锗探测器距离出现在复用函数导数的最大值处。此外，我们报告说，通过使用自适应系统设计可以实现更好的任务性能，其中锗探测器距离可以随投影角而变化。最后，在我们的比较研究中，我们发现 Wiener 估计器优于传统的感兴趣区域估计器。
我们的工作表明，这种优化方法具有快速高效地探索广阔参数空间的潜力，为竞争因素的行为提供了深入的了解，否则使用其他现有方法很难计算和研究这些因素。
