Department of Radiology, Johns Hopkins University, Baltimore, Maryland 21287-0859, USA.
Med Phys. 2013 Aug;40(8):082504. doi: 10.1118/1.4813297.
Post-therapy quantitative 90Y bremsstrahlung single photon emission computed tomography (SPECT) has shown great potential to provide reliable activity estimates, which are essential for dose verification. Typically 90Y imaging is performed with high- or medium-energy collimators. However, the energy spectrum of 90Y bremsstrahlung photons is substantially different than typical for these collimators. In addition, dosimetry requires quantitative images, and collimators are not typically optimized for such tasks. Optimizing a collimator for 90Y imaging is both novel and potentially important. Conventional optimization methods are not appropriate for 90Y bremsstrahlung photons, which have a continuous and broad energy distribution. In this work, the authors developed a parallel-hole collimator optimization method for quantitative tasks that is particularly applicable to radionuclides with complex emission energy spectra. The authors applied the proposed method to develop an optimal collimator for quantitative 90Y bremsstrahlung SPECT in the context of microsphere radioembolization.
To account for the effects of the collimator on both the bias and the variance of the activity estimates, the authors used the root mean squared error (RMSE) of the volume of interest activity estimates as the figure of merit (FOM). In the FOM, the bias due to the null space of the image formation process was taken in account. The RMSE was weighted by the inverse mass to reflect the application to dosimetry; for a different application, more relevant weighting could easily be adopted. The authors proposed a parameterization for the collimator that facilitates the incorporation of the important factors (geometric sensitivity, geometric resolution, and septal penetration fraction) determining collimator performance, while keeping the number of free parameters describing the collimator small (i.e., two parameters). To make the optimization results for quantitative 90Y bremsstrahlung SPECT more general, the authors simulated multiple tumors of various sizes in the liver. The authors realistically simulated human anatomy using a digital phantom and the image formation process using a previously validated and computationally efficient method for modeling the image-degrading effects including object scatter, attenuation, and the full collimator-detector response (CDR). The scatter kernels and CDR function tables used in the modeling method were generated using a previously validated Monte Carlo simulation code.
The hole length, hole diameter, and septal thickness of the obtained optimal collimator were 84, 3.5, and 1.4 mm, respectively. Compared to a commercial high-energy general-purpose collimator, the optimal collimator improved the resolution and FOM by 27% and 18%, respectively.
The proposed collimator optimization method may be useful for improving quantitative SPECT imaging for radionuclides with complex energy spectra. The obtained optimal collimator provided a substantial improvement in quantitative performance for the microsphere radioembolization task considered.
治疗后定量 90Y 韧致辐射单光子发射计算机断层扫描（SPECT）显示出提供可靠活动估计的巨大潜力，这对于剂量验证至关重要。通常使用高或中能准直器进行 90Y 成像。然而，90Y 韧致辐射光子的能谱与这些准直器的典型能谱有很大不同。此外，剂量学需要定量图像，准直器通常不适用于此类任务。优化用于 90Y 成像的准直器既新颖又重要。传统的优化方法不适用于 90Y 韧致辐射光子，因为它们具有连续且广泛的能量分布。在这项工作中，作者开发了一种用于定量任务的平行孔准直器优化方法，该方法特别适用于具有复杂发射能谱的放射性核素。作者应用所提出的方法开发了一种用于微球体放射性栓塞治疗中定量 90Y 韧致辐射 SPECT 的最佳准直器。
为了考虑准直器对活动估计的偏差和方差的影响，作者使用感兴趣区活动估计的均方根误差（RMSE）作为衡量标准（FOM）。在 FOM 中，考虑了图像形成过程的零空间引起的偏差。RMSE 由质量的倒数加权，以反映剂量学的应用；对于不同的应用，可以采用更相关的加权。作者提出了一种准直器参数化方法，该方法便于纳入确定准直器性能的重要因素（几何灵敏度、几何分辨率和隔室穿透分数），同时保持描述准直器的自由参数数量较小（即两个参数）。为了使定量 90Y 韧致辐射 SPECT 的优化结果更加通用，作者模拟了肝脏中各种大小的多个肿瘤。作者使用数字体模真实地模拟了人体解剖结构，并使用以前验证过的、计算效率高的方法模拟了图像退化效应，包括物体散射、衰减和完整的准直器-探测器响应（CDR）。用于建模方法的散射核和 CDR 函数表是使用以前验证过的蒙特卡罗模拟代码生成的。
获得的最佳准直器的孔长度、孔直径和隔室厚度分别为 84、3.5 和 1.4mm。与商业高能通用准直器相比，最佳准直器分别提高了 27%和 18%的分辨率和 FOM。
所提出的准直器优化方法可能有助于改善具有复杂能谱的放射性核素的定量 SPECT 成像。获得的最佳准直器在考虑的微球体放射性栓塞治疗任务中提供了定量性能的大幅提高。
