Dynamic B₀ shimming for multiband imaging using high order spherical harmonic shims

Abstract

Purpose: To describe and implement a strategy for dynamic slice-by-slice and multiband B 0 shimming using spherical harmonic shims in the human brain at 7T. Theory: For thin axial slices, spherical harmonic shims can be divided into pairs of shims (z-degenerate and non-z-degenerate) that are spatially degenerate, such that only ½ of the shims (non-z-degenerate) are required for single slice optimizations. However, when combined, the pairs of shims can be used to simultaneously generate the same in-plane symmetries but with different amplitudes as a function of their z location. This enables multiband shimming equivalent to that achievable by single slice-by-slice optimization. Methods: All data were acquired at 7T using a spherical harmonic shim insert enabling shimming up through 4th order with two additional 5th order shims (1st-4th+). Dynamic shim updating was achieved using a 10A shim power supply with 2 ms ramps and constrained optimizations to minimize eddy currents. Results: In groups of eight subjects, we demonstrated that: 1) dynamic updating using 1st-4th+ order shims reduced the SD of the B 0 field over the whole brain from 32.4 ± 2.6 and 24.9 ± 2 Hz with 1st-2nd and 1st-4th+ static global shimming to 15.1 ± 1.7 Hz; 2) near equivalent performance was achieved when dynamically updating only the non-z-degenerate shims (14.3 ± 1.5 Hz), or when a using multiband shim factor of 2, MB s = 2, and all shims (14.4 ± 2.0 Hz). Conclusion: High order spherical harmonics provide substantial improvements over static global shimming and enable dynamic multiband shimming with near equivalent performance to that of dynamic slice-by-slice shimming. This reduces distortion in echo planar imaging.

An orthogonal shim coil for 3T brain imaging

“…However, multi‐band imaging requires several slices to be shimmed at the same time, posing a different shim challenge from the 2 cases considered here. There have been several explorations of multi‐coil and SH coil performance for multi‐band acquisitions . We plan to explore how our shim coil array performs in multi‐band imaging in the near future.…”

7 T Musculoskeletal MRI

“…43 Static Field (B 0 ) Variations Magnetic field strength and, thus, 1 H Larmor frequency differences between materials of different magnetic susceptibility linearly scale with the field strength, providing the well-known advantages 44 and challenges, 45 for example, for functional and susceptibility-weighted brain imaging at 7 T. 46 In the knee, this may contribute to the high sensitivity, with which 7 T MRI can detect calcified depositions in cartilage and menisci 19 and would likely help to increase the sensitivity in phase and susceptibility-weighted imaging 47 or T2* mapping 48 at 7 T. Homogenization of the magnetic field before acquisition and off-center imaging, 49 however, remains a challenge 50,51 that keeps being tackled most dynamically for head imaging. [52][53][54] Metal-induced susceptibility artifacts are particularly large at 7 T compared with those at lower field strength. 55,56…”

Efficient eddy current characterization using a 2D image‐based sampling scheme and a model‐based fitting approach

“…This approach was only feasible because the small‐diameter shims of the VHOS system did not generate significant first‐order EC fields. The same effects have been observed in a preliminary study on a similar setup, 35 but might not hold true for other shim systems 12 . However, the exclusion is not expected to be necessary on the conventional large‐diameter systems, as they appear to have significantly lower static cross‐terms and the ability to generate first‐order pre‐emphasis fields.…”