Artifacts induced by concomitant magnetic field in fast spin-echo imaging

It has been observed that fast spin-echo (FSE) images with a large field of view (>40 cm) in certain directions exhibit unusual ghosting artifacts that cannot be eliminated with existing ghost removal methods. These artifacts have been related to a higher-order magnetic field perturbation (known as the concomitant field, or Maxwell field) concomitant to the linear imaging gradient, in accordance with the Maxwell equations <(del)over bar> . (B) over bar = 0 and <(del)over bar> x (B) over bar approximate to (0) over bar. Several methods have been developed to eliminate or minimize the effects of the concomitant magnetic field by redesigning the FSE pulse sequences. In the slice-selection direction, 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 shown how crusher gradients can be reshaped to nullify the phase due to the concomitant field. In the phase-encoding direction, the gradient amplitude is reduced and its duration is prolonged. Artifacts due to the readout gradient are eliminated by reshaping the prephasing lobe, while keeping its area fixed. In all the three directions, the gradient waveforms are adjusted so that they have minimal overlap. Selected methods have been implemented on a clinical scanner, and typically reduce the ghost intensities in phantom and human images by a factor of 3.