Study on the radiofrequency transparency of electrically floating and ground PET inserts in a 3 T clinical MRI system
Study on the radiofrequency transparency of electrically floating and ground PET inserts in a 3 T clinical MRI system
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3 T 临床 MRI 系统中电浮动和磨削 PET 插入物的射频透明度研究
DOI：
10.1002/mp.15588
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发表时间：
2022
期刊：
影响因子：
--
通讯作者：
Taiga Yamaya
中科院分区：
文献类型：
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作者：
Md Shahadat Hossain Akram;Takayuki Obata;Fumihiko Nishikido;Taiga Yamaya
目的用于磁共振成像 (MRI) 系统的正电子发射断层扫描 (PET) 插件将 MRI 系统的射频 (RF) 内置人体线圈用作发射器，设计为射频透明，因为线圈位于射频屏蔽 PET 环外部。这种方法降低了与在 PET 环内部实施发射线圈相关的设计复杂性（例如，大 PET 环直径）。然而，由于 PET 插入物的射频屏蔽，实现成像感兴趣区域 (ROI) 所需的场传输变得具有挑战性。在本研究中，考虑射频屏蔽的两种电气配置，即电气浮动和接地配置，使用模块化射频屏蔽 PET 插入物来研究射频透明度。目的是找出这两种配置的差异、优点和缺点。方法八个铜屏蔽 PET 探测器模块（模块间间隙：3 毫米）定向为圆柱形，内径为 234 毫米。每个 PET 模块包括四层镥钇氧原硅酸盐闪烁晶体块和前端读出电子器件。射频屏蔽双绞线电缆用于将前端电子设备与位于 MRI 室外的电源和 PET 数据采集系统连接起来。在接地配置中，探测器和电缆屏蔽层均连接至 MRI 系统的射频接地。在浮动配置中，仅 PET 模块的 RF 屏蔽与 RF 接地隔离。在 3 T 临床 MRI 系统中使用两个圆柱形均质体模进行实验，其中内置人体 RF 线圈（直径 700 mm、长度 540 mm 的圆柱形体积线圈）用作收发器。结果对于两种 PET 配置，RF 和 MR 成像性能均低于仅 MRI 的情况，并且 MRI 系统提供的比吸收比 (SAR) 值几乎是其两倍。浮动 PET 配置的射频均匀性、场强以及 MR 图像的信噪比 (SNR) 大多高于地面 PET 配置。然而，对于 125 毫米的较短轴向视场 (FOV)，两种配置提供几乎相同的性能和高射频均匀性（例如 76 ± 10%）。此外，对于两种 PET 配置，MR 成像需要更大 56 ± 6% 的 RF 脉冲幅度。增加的功率大部分以屏蔽射频涡流的形式被导电屏蔽吸收；因此，仅模型中的 SAR 值估计接近于仅 MRI 的值。 结论 浮动 PET 配置在所有实验设置下都显示出更高的 RF 透明度。对于 125 毫米相对较短的轴向 FOV，接地配置也表现良好，这表明采用传统设计（例如接地配置）的射频可穿透 PET 插入物也可能成为可能。然而，一些设计修改（例如，更宽的模块间间隙以及在 PET 插入物内使用 RF 接收器线圈）应将 RF 性能提高到仅 MRI 情况的水平。
PurposeThe positron emission tomography (PET) insert for a magnetic resonance imaging (MRI) system that implements the radiofrequency (RF) built‐in body coil of the MRI system as a transmitter is designed to be RF‐transparent, as the coil resides outside the RF‐shielded PET ring. This approach reduces the design complexities (e.g., large PET ring diameter) related to implementing a transmit coil inside the PET ring. However, achieving the required field transmission into the imaging region of interest (ROI) becomes challenging because of the RF shield of the PET insert. In this study, a modularly RF‐shielded PET insert is used to investigate the RF transparency considering two electrical configurations of the RF shield, namely the electrical floating and ground configurations. The purpose is to find the differences, advantages and disadvantages of these two configurations.MethodsEight copper‐shielded PET detector modules (intermodular gap: 3 mm) were oriented cylindrically with an inner diameter of 234 mm. Each PET module included four‐layer Lutetium‐yttrium oxyorthosilicate scintillation crystal blocks and front‐end readout electronics. RF‐shielded twisted‐pair cables were used to connect the front‐end electronics with the power sources and PET data acquisition systems located outside the MRI room. In the ground configuration, both the detector and cable shields were connected to the RF ground of the MRI system. In the floating configuration, only the RF shields of the PET modules were isolated from the RF ground. Experiments were conducted using two cylindrical homogeneous phantoms in a 3 T clinical MRI system, in which the built‐in body RF coil (a cylindrical volume coil of diameter 700 mm and length 540 mm) was implemented as a transceiver.ResultsFor both PET configurations, the RF and MR imaging performances were lower than those for the MRI‐only case, and the MRI system provided specific absorption ratio (SAR) values that were almost double. The RF homogeneity and field strength, and the signal‐to‐noise ratio (SNR) of the MR images were mostly higher for the floating PET configuration than they were for the ground PET configuration. However, for a shorter axial field‐of‐view (FOV) of 125 mm, both configurations offered almost the same performance with high RF homogeneities (e.g., 76 ± 10%). Moreover, for both PET configurations, 56 ± 6% larger RF pulse amplitudes were required for MR imaging purposes. The increased power is mostly absorbed in the conductive shields in the form of shielding RF eddy currents; as a result, the SAR values only in the phantoms were estimated to be close to the MRI‐only values.ConclusionsThe floating PET configuration showed higher RF transparency under all experimental setups. For a relatively short axial FOV of 125 mm, the ground configuration also performed well which indicated that an RF‐penetrable PET insert with the conventional design (e.g., the ground configuration) might also become possible. However, some design modifications (e.g., a wider intermodular gap and using the RF receiver coil inside the PET insert) should improve the RF performance to the level of the MRI‐only case.