Department of Biomedical Engineering, Case Western Reserve University, Cleveland, Ohio, USA.
Vanderbilt University Institute of Imaging Science, Nashville, Tennessee, USA.
NMR Biomed. 2024 Dec;37(12):e5245. doi: 10.1002/nbm.5245. Epub 2024 Aug 26.
Conventional gradient systems have several weaknesses including high cost and bulk. As a step towards addressing these while providing new degrees of freedom for spatial encoding and system design in Magnetic Resonance Imaging (MRI), a radio frequency (RF) gradient encoding system and pulse sequence for phase encoding using the Bloch-Siegert (BS) shift were developed. Optimized BS spatial encoding coils with bucking windings (counter-wound loops) were designed and constructed, along with compatible homogeneous imaging coils for excitation and signal reception. Two coil systems were developed: one for phantom imaging and a second for human wrist imaging. BS phase-encoded imaging and BS RF pulse simulations were performed. Pulse sequences were designed for linear stepping in k-space and implemented on a 47.5-mT scanner to image resolution phantoms in both coil setups. Reconstructions were performed using both the full -based encoding fields for each BS pulse amplitude and using inverse discrete Fourier transforms. A gradient was used for frequency encoding during signal readout, and the third axis was projected. Specific absorption ratio (SAR) calculations were performed for the wrist coil to determine the safety of BS-based RF encoding for fields in the low field MRI regime. The optimized RF spatial encoding coils resulted in higher linearity ( and 0.9921 in the phantom and wrist coils, respectively) than coils used in previous work. The phantom and wrist imaging coils were validated in simulations and experimentally to produce a peak G and 0.8 G with 12-W input power, respectively, in the field-of-view (length = 11 cm) used for imaging. Nominal imaging resolutions of 5.22 and 7.21 mm were, respectively, achieved by the two-coil systems in the RF phase-encoded dimension. Coil systems, pulse sequences, and image reconstructions were developed for linear RF phase encoding using the BS shift and validated using a 47.5-mT open low field scanner, establishing a key component required for gradient-free imaging at low field strengths.
传统的梯度系统存在成本高、体积大等缺点。为了解决这些问题，同时为磁共振成像（MRI）中的空间编码和系统设计提供新的自由度，我们开发了一种使用 Bloch-Siegert（BS）偏移的射频（RF）梯度编码系统和相位编码脉冲序列。设计并构建了带有补偿绕组（反向缠绕的环路）的优化 BS 空间编码线圈，以及用于激励和信号接收的兼容均匀成像线圈。开发了两种线圈系统：一种用于体模成像，另一种用于人手腕成像。进行了 BS 相位编码成像和 BS RF 脉冲模拟。设计了用于 k 空间线性步进的脉冲序列，并在 47.5-mT 扫描仪上实现，以在两个线圈设置中对分辨率体模进行成像。使用每个 BS 脉冲幅度的全 - 编码场和逆离散傅里叶变换进行重建。在信号读出期间使用梯度进行频率编码，并投影第三轴。对手腕线圈进行了特定吸收率（SAR）计算，以确定在低场 MRI 范围内基于 BS 的 RF 编码的安全性。优化的 RF 空间编码线圈具有更高的线性度（分别为 和 0.9921，在体模和手腕线圈中），比以前工作中使用的线圈更高。在模拟和实验中验证了体模和手腕成像线圈，分别在视场（长度 = 11 厘米）中产生 12-W 输入功率下的峰值 和 0.8 G ，用于成像。两个线圈系统在 RF 相位编码维度上分别实现了标称成像分辨率为 5.22 和 7.21 毫米。使用 BS 偏移开发了用于线性 RF 相位编码的线圈系统、脉冲序列和图像重建，并使用 47.5-mT 开放式低场扫描仪进行了验证，为在低场强下实现无梯度成像建立了关键组件。
