Center for Medical Physics and Biomedical Engineering, Medical University Vienna, Vienna, Austria.
Department of Radiation Oncology, Medical University Vienna, Vienna, Austria.
Med Phys. 2022 Apr;49(4):2366-2372. doi: 10.1002/mp.15586. Epub 2022 Mar 8.
MR-based methods for attenuation correction (AC) in PET/MRI either neglect attenuation of bone, or use MR-signal derived information about bone, which leads to a bias in quantification of tracer uptake in PET. In a previous study, we presented a PET/MRI specific MR coil with an integrated transmission source (TX) system allowing for direct measurement of attenuation. In phantom measurements, this system successfully reproduced the linear attenuation coefficient of water.
The purpose of this study is to validate the TX system in a clinical setting using animals and to show its applicability compared to standard clinical methods.
As test subject, a 15-kg piglet was injected with 53 MBq of 18F-NaF. The μ-map obtained with the TX system and the reconstructed activity distribution were compared to four established AC methods: a Dixon sequence, an ultra-short echo time (UTE) sequence, a CT scan, and a 511 keV transmission scan using a Siemens ECAT EXACT HR+ as the reference. The PET/MRI measurements were performed on a Siemens Biograph mMR to obtain the μ-map using the TX system as well as the Dixon and UTE sequence directly followed by the CT and ECAT measurements.
The reconstructed activity distribution using the TX system for AC showed similar results compared to the reference (<5% difference in hot regions) and outperformed the MR-based methods as implemented in the PET/MRI system (<10% difference in hot regions). However, the additional hardware of the TX system adds complexity to the acquisition process.
Our porcine study demonstrates the feasibility of post-injection transmission scans using the developed TX system in a clinical setting. This makes it a useful tool for PET/MRI in cases where transmission information is needed for AC. Potential applications are studies using larger animals where state-of-the-art atlas-based or artificial intelligence AC methods are not available.
在 PET/MRI 中，基于磁共振（MR）的衰减校正（AC）方法要么忽略了骨的衰减，要么使用了关于骨的 MR 信号衍生信息，这导致了 PET 中示踪剂摄取的定量存在偏差。在之前的研究中，我们提出了一种具有集成透射源（TX）系统的专用 PET/MRI 磁共振线圈，该系统可用于直接测量衰减。在体模测量中，该系统成功地复制了水的线性衰减系数。
本研究的目的是在临床环境中使用动物验证 TX 系统，并展示其与标准临床方法相比的适用性。
作为测试对象，一只 15 公斤的小猪注射了 53MBq 的 18F-NaF。使用 TX 系统获得的 μ 图和重建的活性分布与四种已建立的 AC 方法进行了比较：Dixon 序列、超短回波时间（UTE）序列、CT 扫描和使用西门子 ECAT EXACT HR+作为参考的 511keV 透射扫描。PET/MRI 测量是在西门子 Biograph mMR 上进行的，使用 TX 系统获得 μ 图，并直接在 Dixon 和 UTE 序列之后进行 CT 和 ECAT 测量。
使用 TX 系统进行 AC 的重建活性分布与参考结果相似（热点区域差异<5%），并且优于 PET/MRI 系统中实施的基于 MR 的方法（热点区域差异<10%）。然而，TX 系统的附加硬件增加了采集过程的复杂性。
我们的猪研究证明了在临床环境中使用开发的 TX 系统进行注射后透射扫描的可行性。这使其成为需要用于 AC 的透射信息的 PET/MRI 的有用工具。潜在的应用是在无法使用最先进的基于图谱或人工智能 AC 方法的情况下，使用较大动物进行的研究。
