Selective RF pulses yield imperfect slice profiles, whose edges are not clearly cut. In multislice technique and with contiguous slices, a selective RF pulse can thus partially excite the adjacent slices. Likewise, if several interlacing slice stacks cross, the zone of intersection will be partially excited. This will cause a modification in contrast and/or a loss of signal through partial saturation in the slice or zone of intersection. These phenomena are even more pronounced with pulses of 180° (inversion recovery, fast spin echo).
The solution consists in spacing the slices by adding an interval between them: the slices are no longer contiguous. It is also possible to interlace multislice acquisition to avoid imaging the adjacent slices with the same repetition. Where it is vital to visualize the whole volume with no wasted time, it is preferable to use 3D imaging.
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