The limitations of NMR recalled-echo imaging techniques

An echo-planar pulse sequence can encode a single free induction signal with enough information to reconstruct a two- or three-dimensional image. Echo-planar pulse sequences involve the production of a train of echoes (recalled echoes) and the most easily implemented variant achieves this by repeated reversal of one gradient field. Alternate echoes are time reversed and failure to take this into account will result in aliasing. A modified pulse sequence is presented and analyzed using a simple model. The effects of transverse relaxation and magnetic field inhomogeneities are presented in terms of this model. Reversal of sample order in alternate echoes produced by the modified sequence eliminates aliasing associated with the time reversal without an increase in data collection time (or degradation of resolution). Inhomogeneities are likely to limit practically achievable resolution. It is shown that the method is intrinsically less efficient than other Fourier imaging techniques due to the finite gradient reversal time. Practical details of the implementation of the method are discussed and crude phantom images are presented.