Marin I, Rydèn T, Van Essen M, Svensson J, Gracheva N, Köster U, Zeevaart J R, van der Meulen N P, Müller C, Bernhardt P
Department of Radiation Physics, Institution of Clinical Science, Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden.
Department of Medical Physics and Bioengineering, Sahlgrenska University Hospital, Gula Stråket 2B, 413 45, Gothenburg, Sweden.
EJNMMI Phys. 2020 Jul 1;7(1):45. doi: 10.1186/s40658-020-00314-x.
It has been proposed, and preclinically demonstrated, that Tb is a better alternative to Lu for the treatment of small prostate cancer lesions due to its high emission of low-energy electrons. Tb also emits photons suitable for single-photon emission computed tomography (SPECT) imaging. This study aims to establish a SPECT protocol for Tb imaging in the clinic.
Optimal settings using various γ-camera collimators and energy windows were explored by imaging a Jaszczak phantom, including hollow-sphere inserts, filled with Tb. The collimators examined were extended low-energy general purpose (ELEGP), medium-energy general purpose (MEGP), and low-energy high resolution (LEHR), respectively. In addition, three ordered subset expectation maximization (OSEM) algorithms were investigated: attenuation-corrected OSEM (A-OSEM); attenuation and dual- or triple-energy window scatter-corrected OSEM (AS-OSEM); and attenuation, scatter, and collimator-detector response-corrected OSEM (ASC-OSEM), where the latter utilized Monte Carlo-based reconstruction. Uniformity corrections, using intrinsic and extrinsic correction maps, were also investigated. Image quality was assessed by estimated recovery coefficients (RC), noise, and signal-to-noise ratio (SNR). Sensitivity was determined using a circular flat phantom.
The best RC and SNR were obtained at an energy window between 67.1 and 82.1 keV. Ring artifacts, caused by non-uniformity, were removed with extrinsic uniformity correction for the energy window between 67.1 and 82.1 keV, but not with intrinsic correction. Analyzing the lower energy window between 48.9 and 62.9 keV, the ring artifacts remained after uniformity corrections. The recovery was similar for the different collimators when using a specific OSEM reconstruction. Recovery and SNR were highest for ASC-OSEM, followed by AS-OSEM and A-OSEM. When using the optimized parameter setting, the resolution of Tb was higher than for Lu (8.4 ± 0.7 vs. 10.4 ± 0.6 mm, respectively). The sensitivities for Tb and Lu were 7.41 and 8.46 cps/MBq, respectively.
SPECT with high resolution is feasible with Tb; however, extrinsic uniformity correction is recommended to avoid ring artifacts. The LEHR collimator was the best choice of the three tested to obtain a high-resolution image. Due to the complex emission spectrum of low-energy photons, window-based scatter correction had a minor impact on the image quality compared to using attenuation correction only. On the other hand, performing attenuation, scatter, and collimator-detector correction clearly improved image quality. Based on these data, SPECT-based dosimetry for Tb-labeled radiopharmaceuticals is feasible.
有人提出并在临床前证明，由于铽（Tb）能高发射低能电子，因此在治疗小前列腺癌病灶方面，它是镥（Lu）的更好替代物。Tb还发射适用于单光子发射计算机断层扫描（SPECT）成像的光子。本研究旨在建立临床中Tb成像的SPECT方案。
通过对装有Tb的Jaszczak体模（包括空心球插入物）进行成像，探索使用各种γ相机准直器和能量窗的最佳设置。所检查的准直器分别为扩展型低能通用型（ELEGP）、中能通用型（MEGP）和低能高分辨率型（LEHR）。此外，研究了三种有序子集期望最大化（OSEM）算法：衰减校正OSEM（A - OSEM）；衰减以及双能或三能窗散射校正OSEM（AS - OSEM）；以及衰减、散射和准直器 - 探测器响应校正OSEM（ASC - OSEM），其中后者利用基于蒙特卡罗的重建。还研究了使用固有和外部校正图进行均匀性校正。通过估计恢复系数（RC）、噪声和信噪比（SNR）评估图像质量。使用圆形平面体模确定灵敏度。
在67.1至82.1 keV的能量窗处获得了最佳的RC和SNR。对于67.1至82.1 keV的能量窗，通过外部均匀性校正消除了由不均匀性引起的环形伪影，但固有校正未消除。分析48.9至62.9 keV的较低能量窗时，均匀性校正后环形伪影仍然存在。使用特定的OSEM重建时，不同准直器的恢复情况相似。ASC - OSEM的恢复和SNR最高,其次是AS - OSEM和A - OSEM。使用优化的参数设置时，Tb的分辨率高于Lu（分别为8.4±0.7与10.4±0.6 mm）。Tb和Lu的灵敏度分别为7.41和8.46 cps/MBq。
使用Tb进行高分辨率SPECT是可行的；然而，建议进行外部均匀性校正以避免环形伪影。LEHR准直器是所测试的三种中获得高分辨率图像的最佳选择。由于低能光子的发射光谱复杂，与仅使用衰减校正相比，基于窗的散射校正对图像质量的影响较小。另一方面，进行衰减、散射和准直器 - 探测器校正明显提高了图像质量。基于这些数据，基于SPECT的Tb标记放射性药物剂量测定是可行的。
