A couple of authors from the French Neurospin team -- known for their 20-year efforts on 11.7T brain MRI -- recently published in MAGMA: official journal of ESMRMB, a full-length article on physical limits of brain B0 shimming. The title is "Physical limits to human brain B0 shimming with spherical harmonics, engineering implications thereof" and the full text is freely available here. An interesting, eye-catching sentence is "We establish a ~13Hz inhomogeneity hard shim limit at 7T for whole brain SH (spherical harmonic) shimming, ... only be attained at shimming degree higher than 90."
Since the B0 inhomogeneity is generated largely by the surface current in the brain itself, it is well-known that external shim current cannot completely cancel such fields. These authors tested this theoretical limit based on experimental B0 map data from 100 subjects.
In terms of citation, the paper prominently cited the SKKU CIEL group's 2020 paper on head-tilted imaging by Yoo et al., dedicating a full sentence which mentioned the latter as an example of non-conventional way of pre-frontal cortex shimming.
MAGMA had another article on brain B0 shimming in the same batch of (total 19) articles called topical collection: Basic Science - Engineering. In this other paper, the authors (half of whom were apparently from the Boston team promoting combined shim + RF array) basically said that a combined local shim + RF phased array is too complicated, and a simpler 8 channel shim array can already do very well in 7T brain shimming. B0 shimming lives on!
