Dynamic High-Order Shim System for Improved Neuro-MRI

Abstract The purpose of this proposal to demonstrate the feasibility of building a practical shim sub-system based on a minimally interacting in-bore shim coil using a highly controllable power array for use in commercial MRI scanners for multi-band neuroimaging and spectroscopy applications at high fields.

Over the past several years, the importance of improved B0 shimming for echo planar imaging (EPI) has been recognized by several groups. Technical developments have focused on ways of providing higher spatial frequency corrections and higher strength of local B0 corrections. Two methods of accomplishing those goals

were applied: 1) multi-coil (MC) arrays frequently utilizing DC-coupled RF coils or 2) very-high order shim (VHOS) inserts based on spherical harmonic (SH) symmetries. Also, for the human brain, optimizing the B0 shim corrections on a slice-by-slice (SBS) basis was demonstrated to significantly improve homogeneity. Because of

this, several groups have developed dynamic updating approaches, including strategies to mitigate artifacts due to eddy-current effects associated with dynamic shim updating (DSU). It has been demonstrated that the use of VHOS inserts based on 4th order SH can provide superior shim performance to sparse MC arrays for both static

and dynamic shimming. It has also been demonstrated that theoretically and experimentally VHOS inserts can achieve MB=2 with equivalent homogeneity to SBS with significant gains over whole brain shimming maintained up to MB=4. Thus, a new VHOS insert technology capable of distinct corrections at up to 3 or 4 spatially disparate

slice locations simultaneously with equivalent performance to SBS shimming enabled by DSU technology will greatly improve performance of MB EPI and other sensitive techniques. To achieve the goal of up to MB=4 we will leverage our recent work on second generation 7T scanners. The goal of this Phase I proposal is to demonstrate an optimized prototype of easily installable digitally controlled,

real-time multiband shimming system for Siemens Terra 7T implementation with commercialization in Phase II.