Combined shim-RF for improved high field MRI

描述（由申请人提供）：B0 匀场效果不佳仍然是在超高场获取高分辨率、可重复 MRI 扫描的最大障碍之一。对于功能性、灌注和扩散成像所需的平面回波成像 (EPI) 等快速序列来说，这个问题尤其严重。例如，B0 场的空间变化会导致 几何失真和信号丢失阻碍了对靠近窦腔和耳道的人脑区域进行准确的 EPI 采集。标准二阶和三阶球谐匀场可以提供帮助，但空间阶数不足以完全减轻磁化率场。这项工作将同时 B0 匀场和射频 (RF) 接收结合在一个环路中，用于匀场场的接收和应用。紧密配合的结构优化了这两个需要集成的关键组件的性能。通过将 32 个匀场环组成的阵列放置在头部一厘米范围内，我们可以首次提供高效的高阶匀场校正，以改善超高场脑成像。我们新颖的电路设计使用相同的导电环路来承载射频和直流电流。这些环位于我们之前用于 32 通道接收阵列的紧身头盔上。该设计能够实现高阶多线圈匀场以消除 B0 不均匀性，同时还提供与紧密配合的射频接收线圈阵列相关的高信噪比 (SNR)。通过将这两项任务结合在一个导体中，我们利用了这样一个事实：匀场线圈和射频线圈在尽可能靠近身体时表现最佳。我们证明，即使在承载直流匀场电流时，匀场/射频阵列原型线圈也能提供与标准环路设计等效的接收信噪比。作为敏感性问题的补救措施，所提出的垫片/射频阵列有望在超高场的临床和神经科学成像中得到广泛应用。 7T 神经成像的原理证明也将成为将该技术扩展到身体其他部位的第一步。

DESCRIPTION (provided by applicant): Poor B0 shimming remains one of the biggest obstacles to acquiring high-resolution, repeatable MRI scans at ultra-high field. The problem is particularly acute for fast sequences such as the echo planar imaging (EPI) needed for functional, perfusion and diffusion imaging. For instance, spatial variations in the B0 field cause geometric distortion and signal loss that prevents accurate EPI acquisition of human brain regions near the sinus cavities and ear canals. Standard second and third order spherical harmonic shims can help but have insufficient spatial orders to fully mitigate the susceptibility fields. This work combines simultaneous B0 shimming and radiofrequency (RF) reception in a single loop for both reception and application of the shim fields. The close-fitting structure optimizes performance of both of these critical components necessitating their integration. By placing an array of 32 shim loops within a cm of the head, we can provide, for the first time, efficient high order shim corrections for improved ultra-high field brain imaging. Our novel circui design uses the same conducting loop to carry both RF and DC currents. The loops are positioned on a tight-fitting helmet of the type we have previously used for 32ch receive arrays. This design enables both higher-order multi-coil shimming to cancel B0 inhomogeneity while also providing the high signal-to-noise ratio (SNR) associated with close-fitting arrays of RF receive coils. By combining both tasks in a single conductor, we exploit the fact that both shim coils and RF coils perform best when located as close to the body as possible. We show that the shim/RF array prototype coils provide equivalent receive SNR to a standard loop design, even while carrying DC shim currents. As a remedy for the susceptibility problem, the proposed shim/RF array is expected to find widespread application in both clinical and neuroscientific imaging at ultra high field. Proof of principle in neuroimaging at 7T will also serve as a first stp toward the extension of this technology into other parts of the body.