Department of Radiology and Nuclear Medicine, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX Utrecht, The Netherlands.
Med Phys. 2013 Nov;40(11):112502. doi: 10.1118/1.4823788.
Quantitative imaging of the radionuclide distribution is of increasing interest for microsphere radioembolization (RE) of liver malignancies, to aid treatment planning and dosimetry. For this purpose, holmium-166 ((166)Ho) microspheres have been developed, which can be visualized with a gamma camera. The objective of this work is to develop and evaluate a new reconstruction method for quantitative (166)Ho SPECT, including Monte Carlo-based modeling of photon contributions from the full energy spectrum.
A fast Monte Carlo (MC) simulator was developed for simulation of (166)Ho projection images and incorporated in a statistical reconstruction algorithm (SPECT-fMC). Photon scatter and attenuation for all photons sampled from the full (166)Ho energy spectrum were modeled during reconstruction by Monte Carlo simulations. The energy- and distance-dependent collimator-detector response was modeled using precalculated convolution kernels. Phantom experiments were performed to quantitatively evaluate image contrast, image noise, count errors, and activity recovery coefficients (ARCs) of SPECT-fMC in comparison with those of an energy window-based method for correction of down-scattered high-energy photons (SPECT-DSW) and a previously presented hybrid method that combines MC simulation of photopeak scatter with energy window-based estimation of down-scattered high-energy contributions (SPECT-ppMC+DSW). Additionally, the impact of SPECT-fMC on whole-body recovered activities (A(est)) and estimated radiation absorbed doses was evaluated using clinical SPECT data of six (166)Ho RE patients.
At the same noise level, SPECT-fMC images showed substantially higher contrast than SPECT-DSW and SPECT-ppMC+DSW in spheres ≥ 17 mm in diameter. The count error was reduced from 29% (SPECT-DSW) and 25% (SPECT-ppMC+DSW) to 12% (SPECT-fMC). ARCs in five spherical volumes of 1.96-106.21 ml were improved from 32%-63% (SPECT-DSW) and 50%-80% (SPECT-ppMC+DSW) to 76%-103% (SPECT-fMC). Furthermore, SPECT-fMC recovered whole-body activities were most accurate (A(est) = 1.06 × A - 5.90 MBq, R(2) = 0.97) and SPECT-fMC tumor absorbed doses were significantly higher than with SPECT-DSW (p = 0.031) and SPECT-ppMC+DSW (p = 0.031).
The quantitative accuracy of (166)Ho SPECT is improved by Monte Carlo-based modeling of the image degrading factors. Consequently, the proposed reconstruction method enables accurate estimation of the radiation absorbed dose in clinical practice.
放射性核素分布的定量成像对于肝脏恶性肿瘤的微球放射栓塞（RE）越来越感兴趣，有助于治疗计划和剂量测定。为此，已经开发了钬-166（166Ho）微球，可以用伽马相机进行可视化。这项工作的目的是开发和评估一种用于定量（166）Ho SPECT 的新重建方法，包括基于蒙特卡罗的全能谱光子贡献建模。
开发了一种快速蒙特卡罗（MC）模拟器，用于模拟 166Ho 投影图像，并将其纳入统计重建算法（SPECT-fMC）中。在重建过程中，通过蒙特卡罗模拟对从全 166Ho 能谱中采样的所有光子的散射和衰减进行建模。使用预先计算的卷积核对能量和距离相关的准直器-探测器响应进行建模。通过使用六个 166HoRE 患者的临床 SPECT 数据，进行了体模实验，以定量评估 SPECT-fMC 与基于能量窗的方法（SPECT-DSW）校正散射高能光子和之前提出的结合基于 MC 模拟的光电峰散射和基于能量窗的散射高能贡献估计（SPECT-ppMC+DSW）的混合方法相比，在图像对比度、图像噪声、计数误差和活动回收率（ARC）方面的性能。
在相同噪声水平下，与 SPECT-DSW 和 SPECT-ppMC+DSW 相比，SPECT-fMC 图像在直径≥17mm 的球体中显示出更高的对比度。计数误差从 29%（SPECT-DSW）和 25%（SPECT-ppMC+DSW）降低到 12%（SPECT-fMC）。五个 1.96-106.21ml 球形体积的 ARC 从 32%-63%（SPECT-DSW）和 50%-80%（SPECT-ppMC+DSW）提高到 76%-103%（SPECT-fMC）。此外，SPECT-fMC 恢复的全身活动最准确（A（est）=1.06×A-5.90MBq，R（2）=0.97），SPECT-fMC 肿瘤吸收剂量明显高于 SPECT-DSW（p=0.031）和 SPECT-ppMC+DSW（p=0.031）。
通过基于蒙特卡罗的图像降质因素建模，提高了（166）Ho SPECT 的定量准确性。因此，该方法可以在临床实践中准确估计辐射吸收剂量。
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