Biomedical Imaging Laboratory (BIG), Department of Electrical and Computer Engineering, Faculty of Science and Technology, University of Macau, Macau, SAR, China.
Faculty of Health Sciences, University of Macau, Macau SAR, China.
Med Phys. 2018 Nov;45(11):5138-5144. doi: 10.1002/mp.13197. Epub 2018 Oct 10.
Previously we have shown that using sequential CT images is superior to sequential SPECT for nonrigid registration in three-dimensional (3D) targeted radionuclide therapy (TRT) dosimetry. However, sequential CTs are often not available due to radiation concerns. In this paper, we propose a virtual CT (vCT) method for attenuation and scatter correction, image registration, and segmentation for improved dosimetric accuracy with single CT acquisition.
We used a population of nine XCAT phantoms with different In-111 Zevalin biokinetics and anatomical variations for the simulations. An analytical projector was used to simulate sequential SPECT/CT acquisitions for a medium energy general purpose collimator at 1, 12, 24, 72, and 144 h postinjection, modeling attenuation, scatter, and geometric collimator-detector response. The corresponding sequential attenuation maps of the phantoms served as real CT (rCT) images. For vCT generation, we investigated three registration methods, that is, (a) SPECT to SPECT; (b) SPECT to CT, and (c) CT to SPECT, and the optimal time point for single CT acquisition. Difference images and average normalized mean square errors (NMSE) were calculated between different vCTs and their corresponding rCTs. Absorbed dose and dose-volume histograms (DVHs) for critical organs were computed for the rCT, optimized vCT, and conventional single CT (1CT) protocols, respectively, for dosimetric analyses.
For vCT generation, SPECT to SPECT registration with a single CT acquired at the first time point shows the smallest difference and NMSE. For organ absorbed doses, the results of vCT were similar to those of rCT and were superior to 1CT, that is, -0.24 ± 1.56% vs -0.49 ± 1.76% vs -6.37 ± 5.63% for the liver, -1.05 ± 2.89% vs -0.69 ± 2.74% vs -4.87 ± 4.35% for kidneys, respectively. The results of DVHs also showed improvement for all organs using vCTs as compared to the conventional 1CT protocol.
The optimized vCT method can effectively increase the TRT dosimetric results if there is only a single CT available in the sequential imaging protocol, reducing the substantial increase in radiation burden from repeated CT scans.
我们之前已经表明，在三维（3D）靶向放射性核素治疗（TRT）剂量学中，使用连续 CT 图像优于连续 SPECT 进行非刚性配准。然而，由于辐射问题，通常无法获得连续 CT。在本文中，我们提出了一种虚拟 CT（vCT）方法，用于衰减和散射校正、图像配准和分割，以提高单次 CT 采集的剂量学准确性。
我们使用了 9 个具有不同 In-111 Zevalin 生物动力学和解剖学变化的 XCAT 体模进行模拟。使用分析投影器模拟了中等能量通用准直器在注射后 1、12、24、72 和 144 小时的连续 SPECT/CT 采集，模拟了衰减、散射和几何准直器-探测器响应。体模的相应连续衰减图用作真实 CT（rCT）图像。为了生成 vCT，我们研究了三种注册方法，即（a）SPECT 到 SPECT；（b）SPECT 到 CT；（c）CT 到 SPECT，以及单次 CT 采集的最佳时间点。在不同的 vCT 与其相应的 rCT 之间计算了差异图像和平均归一化均方误差（NMSE）。分别为 rCT、优化的 vCT 和常规的单 CT（1CT）方案计算了关键器官的吸收剂量和剂量体积直方图（DVH），以进行剂量学分析。
对于 vCT 生成，使用单次 CT 在第一时间点进行 SPECT 到 SPECT 配准显示出最小的差异和 NMSE。对于器官吸收剂量，vCT 的结果与 rCT 相似，优于 1CT，即肝脏分别为-0.24±1.56%、-0.49±1.76%、-6.37±5.63%，肾脏分别为-1.05±2.89%、-0.69±2.74%、-4.87±4.35%。与传统的 1CT 方案相比，DVH 的结果也显示所有器官均有改善。
如果连续成像方案中只有单次 CT，则优化的 vCT 方法可以有效地提高 TRT 剂量学结果，减少重复 CT 扫描带来的辐射负担的大幅增加。
