  Background: Magnetic field inhomogeneities generate important geometric
distortions in reconstructed echo-planar images. Various procedures were
proposed for correcting these distortions on brain images; yet, few
neuroimaging studies tailored and incorporated the use of these techniques in
spinal cord diffusion MRI. Purpose: We present a comparative evaluation of
distortion correction methods that use the reversed gradient polarity technique
on spinal cord. We propose novel geometric metrics to measure the alignment of
the reconstructed diffusion model with the apparent centerline of the spinal
cord. Subjects: 95 subjects, among which 29 healthy controls and 66 multiple
sclerosis patients. Assessment: Geometric distortions were corrected using 4
state-of-the-art methods. We measured the alignment of the principal direction
of diffusion with the apparent centerline of the spine after correction and the
correlation with the reference anatomical image. Results are computed per
vertebral level, to evaluate the impact on different portions of the spine.
Besides, subjective evaluation of the quality of the correction of healthy
subjects images was performed by three expert raters. Results: As a result of
distortion correction, the diffusion directions are better aligned locally with
the centerline, in particular at both ends of the acquisition window. The
cross-correlation with anatomical image is also improved by Hyperelastic
Susceptibility Artefact Correction (HySCO) and block-matching. The subjective
evaluation for HySCO is significantly better (p < 0.05) than for
Block-Matching; TOPUP performs significantly worse than the three other
methods. Conclusion: Correction based on HySCO provide best results among the
selected methods.
