Biomedical Imaging Laboratory (BIG), Department of Electrical and Computer Engineering, Faculty of Science and Technology, University of Macau, Taipa, Macau SAR, China.
Department of Nuclear Medicine, The Affiliated Hospital of Southwest Medical University, Nuclear Medicine and Molecular Imaging Key Laboratory of Sichuan Province, No. 25, Taiping St., Luzhou, Sichuan, China.
Z Med Phys. 2023 Feb;33(1):54-69. doi: 10.1016/j.zemedi.2022.03.004. Epub 2022 May 27.
Quantitative activity estimation is essential in nuclear medicine imaging. Mismatch between SPECT and CT images at the same imaging time point due to patient movement degrades accuracy in both diagnostic studies and target radionuclide therapy dosimetry. This work aims to study the mismatch effects between CT and SPECT data on attenuation correction (AC), volume-of-interest (VOI) delineation, and registration for activity estimation.
Nine 4D XCAT phantoms were generated at 1, 24, and 144 h post In-111 Zevalin injection, varying in activity distributions, body sizes, and organ sizes. Realistic noisy SPECT projections were generated by an analytical projector and reconstructed with a quantitative OS-EM method. CT images were shifted, corresponding to SPECT images at each imaging time point, from -5 to 5 voxels and also according to a clinical reference. The effect of mismatched AC maps was evaluated using mismatched CT images for AC in SPECT reconstruction while VOIs were mapped out from matched CTs. The effect of mismatched VOI drawings was evaluated using mismatched CTs to map out target organs while using matched CTs for AC. The effect of mismatched CT images for registration was evaluated by registering sequential mismatched CTs to align corresponding SPECT images, with no AC and VOI mismatch. Bi-exponential curve fitting was performed to obtain time-integrated activity (TIA). Organ activity errors (%OAE) and TIA errors (%TIAE) were calculated.
According to the clinical reference, %OAE was larger for organs near ribs for AC effect. For VOI effect, %OAE was larger for small and low uptake organs. For registration effect, %TIAE were larger when mismatch existed in more numbers of SPECT/CT images, while no substantial difference was observed when using mismatched CT at different imaging time points as registration reference. %TIAE was highest for VOI, followed by registration and AC, e.g., 20.62%±8.61%, 9.33%±4.66% and 1.13%±0.90% respectively for kidneys.
The mismatch between CT and SPECT images poses a significant impact on the accuracy of quantitative activity estimation, attributed particularly from VOI delineation errors. It is recommended to perform registration between emission and transmission images at the same time point to ensure diagnostic and dosimetric accuracy.
在核医学成像中，定量活动估计至关重要。由于患者运动导致在同一成像时间点 SPECT 和 CT 图像不匹配，这会降低诊断研究和目标放射性核素治疗剂量学的准确性。本研究旨在研究 CT 和 SPECT 数据在衰减校正 (AC)、感兴趣区 (VOI) 描绘和配准方面的不匹配效应对活性估计的影响。
在注射 In-111 Zevalin 后 1、24 和 144 小时生成了 9 个 4D XCAT 体模，其活性分布、体型和器官大小各不相同。通过解析投影器生成真实噪声 SPECT 投影，并使用定量 OS-EM 方法进行重建。将 CT 图像与 SPECT 图像在每个成像时间点的对应位置错开，从 -5 到 5 个体素，也可以根据临床参考进行错开。通过使用不匹配的 CT 图像进行 SPECT 重建中的 AC 来评估不匹配 AC 图的效果，同时从匹配的 CT 图像中映射出 VOI。通过使用不匹配的 CT 图像来映射目标器官，同时使用匹配的 CT 图像进行 AC，来评估不匹配 VOI 绘图的效果。通过将连续的不匹配 CT 图像注册到对齐相应的 SPECT 图像上来评估不匹配 CT 图像的配准效果，并且不进行 AC 和 VOI 不匹配。通过双指数曲线拟合获得时间积分活性 (TIA)。计算器官活性误差 (%OAE) 和 TIA 误差 (%TIAE)。
根据临床参考，对于 AC 效应，靠近肋骨的器官的 %OAE 较大。对于 VOI 效应，对于小而低摄取的器官，%OAE 较大。对于配准效果，当 SPECT/CT 图像中存在更多不匹配时，%TIAE 较大，而当使用不同成像时间点的不匹配 CT 作为配准参考时，没有观察到实质性差异。对于肾脏，%TIAE 最高，其次是配准和 AC，分别为 20.62%±8.61%、9.33%±4.66%和 1.13%±0.90%。
CT 和 SPECT 图像之间的不匹配对定量活性估计的准确性有重大影响，主要归因于 VOI 描绘误差。建议在同一时间点进行发射和透射图像之间的配准，以确保诊断和剂量学的准确性。
