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.
We propose developing a technology which we hypothesize will substantially improve clinical MRI by allowing higher image resolution and faster imaging methods. This will address two of the most important limitations of MRI;our ability to visualize small and subtle brain pathology, and our ability to maintain patient comfort and acceptance by keeping the imaging time short.
|Stockmann, Jason P; Witzel, Thomas; Keil, Boris et al. (2016) A 32-channel combined RF and B0 shim array for 3T brain imaging. Magn Reson Med 75:441-51|