Department of Advanced Nuclear Medicine Sciences, Institute of Quantum Medical Science, National Institutes for Quantum Science and Technology (QST), 4-9-1 Anagawa, Inage, Chiba, 263-8555, Japan.
Department of Radiology, School of Medicine, Stanford University, Stanford, CA, 94305-5128, USA.
Jpn J Radiol. 2024 Apr;42(4):382-390. doi: 10.1007/s11604-023-01514-y. Epub 2023 Dec 19.
To perform an MRI compatibility study of an RF field-penetrable oval-shaped PET insert that implements an MRI built-in body RF coil both as a transmitter and a receiver.
Twelve electrically floating RF shielded PET detector modules were used to construct the prototype oval PET insert with a major axis of 440 mm, a minor axis of 350 mm, and an axial length of 225 mm. The electric floating of the PET detector modules was accomplished by isolating the cable shield from the detector shield using plastic tape. Studies were conducted on the transmit (B) RF field, the image signal-to-noise ratio (SNR), and the RF pulse amplitude for a homogeneous cylindrical (diameter: 160 mm and length: 260 mm) phantom (NaCl + NiSO solution) in a 3 T clinical MRI system (Verio, Siemens, Erlangen, Germany).
The B maps for the oval insert were similar to the MRI-only field responses. Compared to the MRI-only values, SNR reductions of 51%, 45%, and 59% were seen, respectively, for the spin echo (SE), gradient echo (GE), and echo planar (EPI) images for the case of oval PET insert. Moreover, the required RF pulse amplitudes for the SE, GE, and EPI sequences were, respectively, 1.93, 1.85, and 1.36 times larger. However, a 30% reduction in the average RF reception sensitivity was observed for the oval insert.
The prototype floating PET insert was a safety concern for the clinical MRI system, and this compatibility study provided clearance for developing a large body size floating PET insert for the existing MRI system. Because of the RF shield of the insert, relatively large RF powers compared to the MRI-only case were required. Because of this and also due to low RF sensitivity of the body coil, the SNRs reduced largely.
对一种可穿透射频场的椭圆形 PET 插入物进行 MRI 兼容性研究，该插入物将内置体部射频线圈用作发射器和接收器。
使用 12 个电浮置 RF 屏蔽 PET 探测器模块构建原型椭圆形 PET 插入物，其长轴为 440mm，短轴为 350mm，轴向长度为 225mm。通过使用塑料带将电缆屏蔽与探测器屏蔽隔离，实现 PET 探测器模块的电浮置。在 3T 临床 MRI 系统（西门子公司的 Verio，德国埃尔朗根）中对同一体积（直径：160mm，长度：260mm）圆柱形（NaCl+NiSO 溶液）均匀体模进行了发射（B）射频场、图像信噪比（SNR）和射频脉冲幅度的研究。
椭圆形插入物的 B 图与仅 MRI 场响应相似。与仅 MRI 值相比，椭圆形 PET 插入物的自旋回波（SE）、梯度回波（GE）和回波平面（EPI）图像的 SNR 分别降低了 51%、45%和 59%。此外，SE、GE 和 EPI 序列所需的射频脉冲幅度分别增加了 1.93、1.85 和 1.36 倍。然而，对于椭圆形插入物，平均 RF 接收灵敏度降低了 30%。
原型浮动 PET 插入物对临床 MRI 系统存在安全隐患，本兼容性研究为在现有 MRI 系统上开发大型浮动 PET 插入物提供了许可。由于插入物的 RF 屏蔽，与仅 MRI 情况相比，需要相对较大的 RF 功率。由于这一点，以及由于体部线圈的 RF 灵敏度较低，SNR 大大降低。
