  Obtaining high quality images of the spinal cord with MRI is difficult,
partly due to the fact that the spinal cord is surrounded by a number of
structures that have differing magnetic susceptibility. This causes
inhomogeneities in the magnetic field, which in turn lead to image artifacts.
In order to address this issue, linear compensation gradients can be employed.
The latter can be generated using an MRI scanner's first order gradient coils
and adjusted on a per-slice basis, in order to correct for through-plane ("z")
magnetic field gradients. This approach is referred to as z-shimming. The aim
of this study is two-fold. The first aim was to replicate aspects of a previous
study wherein z-shimming was found to improve image quality in T2*-weighted
echo-planar imaging. Our second aim was to improve upon the z-shimming approach
by including in-plane compensation gradients and adjusting the compensation
gradients during the image acquisition process so that they take into account
respiration-induced magnetic field variations. We refer to this novel approach
as realtime dynamic shimming. Measurements performed in a group of 12 healthy
volunteers at 3 T show improved signal homogeneity along the spinal cord when
using z-shimming. Signal homogeneity may be further improved by including
realtime compensation for respiration-induced field gradients and by also doing
this for gradients along the in-plane axes.
