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Abstract
The presence of the head in an MRI scanner leads to inhomogeneities in the magnetic
field. These cause the 'susceptibility artifacts' of image distortion and signal dropout.
In this paper, we evaluate a technique called passive shimming, which has the potential
to reduce field inhomogeneities and the resultant artifacts. A piece of a magnetically
active material (pyrolytic graphite) is held on the roof of the participant's mouth
by a plastic mouth mould. We evaluate the effects in several different ways. We show
that the presence of a shim reduces field inhomogeneity across much of the brain.
From field maps, we generate simulations of EPI image intensity and BOLD sensitivity.
Both of these are mainly improved by the presence of shim, although there were small
reductions in some regions. Measured EPI image intensity also mostly increased. Finally,
we ran a reward-punishment task in our subjects, and found that the presence of a
shim increased functional sensitivity in the orbitofrontal cortex. Using the BOLD
sensitivity measure, we provide estimates of the improvement to be expected in functional
studies for a range of neural structures. Passive shims are quick to make and reasonably
comfortable to wear, and have substantial potential for researchers investigating
inferior frontal brain regions using MRI.