Multiple-shot echo-planar imaging

After an introduction to Fourier NMR imaging, the effect of magnetic field inhomogeneity on image quality is discussed. The field inhomogeneity leads to a distortion of the image when the reciprocal space is sampled on an oblique plane. This effect is especially strong in echo-planar imaging (EPI), where the entire data matrix is scanned after a single excitation. The distortions can be reduced by dividing the EPI acquisition into several interleaved shots. Multishot EPI is prone to specific artifacts related to resonance offset, signal amplitude variations, gradient reversal, or transverse interference. These are analyzed and illustrated using phantom and human head images. Various methods for avoiding the artifacts are described, including echo time shifting, variable excitation tip angle, one-directional sampling, and the application of steady-state free precession.