Combined shim-RF for improved high field MRI

描述(申请人提供)：糟糕的B0垫片仍然是获得高分辨率、可重复的超高场MRI扫描的最大障碍之一。这个问题对于快速序列来说尤其严重，例如功能成像、灌注成像和扩散成像所需的回声平面成像(EPI)。例如，B0场的空间变化导致 几何失真和信号丢失，阻碍了对鼻窦腔和耳道附近的人脑区域的准确EPI采集。标准的二阶和三阶球谐垫片可以起到帮助作用，但其空间阶数不足以完全缓解磁化率场。这项工作在单个环路中结合了同时的B0填补和射频(RF)接收，用于垫场的接收和应用。紧密配合的结构优化了这两个关键部件的性能，使它们有必要进行集成。通过在头部一厘米内放置32个填充环的阵列，我们可以第一次为改进的超高场脑成像提供高效的高阶填充环校正。我们的新型电路设计使用相同的传导回路来传输射频和直流电流。环路被放置在我们之前用于32路接收阵列的紧贴式头盔上。这种设计使高阶多线圈垫片既能消除B0不均匀，又能提供与RF接收线圈紧密配合阵列相关的高信噪比(SNR)。通过将这两个任务结合在一个导体中，我们利用了这样一个事实，即垫片线圈和射频线圈在尽可能靠近身体的情况下都表现得最好。我们证明，即使在携带直流填补电流的情况下，垫片/射频阵列原型线圈也能提供与标准环路设计相同的接收信噪比。作为一种解决敏感性问题的方法，所提出的填补/射频阵列有望在临床和神经科学的超高场成像中得到广泛的应用。7T神经成像的原理证明也将成为将这项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.