It has been observed that fast spin-echo (FSE) images with a large fie
ld of view (>40 cm) in certain directions exhibit unusual ghosting art
ifacts that cannot be eliminated with existing ghost removal methods.
These artifacts have been related to a higher-order magnetic field per
turbation (known as the concomitant field, or Maxwell field) concomita
nt to the linear imaging gradient, in accordance with the Maxwell equa
tions <(del)over bar> . (B) over bar = 0 and <(del)over bar> x (B) ove
r bar approximate to (0) over bar. Several methods have been developed
to eliminate or minimize the effects of the concomitant magnetic fiel
d by redesigning the FSE pulse sequences. In the slice-selection direc
tion, the gradient waveforms are made symmetrical about the refocusing
RF pulses wherever possible. Surrounding the first refocusing pulse,
such symmetry cannot be achieved due to the slice-refocusing gradient,
which is often combined with the left crusher. In this case, it is sh
own how crusher gradients can be reshaped to nullify the phase due to
the concomitant field. In the phase-encoding direction, the gradient a
mplitude is reduced and its duration is prolonged. Artifacts due to th
e readout gradient are eliminated by reshaping the prephasing lobe, wh
ile keeping its area fixed. In all the three directions, the gradient
waveforms are adjusted so that they have minimal overlap. Selected met
hods have been implemented on a clinical scanner, and typically reduce
the ghost intensities in phantom and human images by a factor of 3.