Two-dimensional spatially-selective RF excitation pulses in echo-planar imaging.

Two-dimensional spatially-selective RF (2DRF) excitation pulses were developed for single-shot echo-planar imaging (EPI) with reduced field of view (FOV) in the phase-encoding direction. The decreased number of k-space lines significantly shortens the length of the EPI echo train. Thus, both gradient-echo and spin-echo 2DRF-EPI images of the human brain at 2.0 T exhibit markedly reduced susceptibility artifacts in regions close to major air cavities. Based on a blipped-planar trajectory, implementation of a typical 2DRF pulse resulted in a 26-ms pulse duration, a 5-mm section thickness, a 40-mm FOV along the phase-encoding direction, and a 200-mm distance of the unavoidable side excitations from the center of the FOV. For the above conditions and at 2 x 2 mm(2) resolution, 2DRF-EPI yielded an echo train length of only 21 ms, as opposed to 102 ms for conventional EPI. This gain in time may be used to achieve higher spatial resolution. For example, spin-echo 2DRF-EPI of a 40-mm FOV at 1 x 1 mm(2) resolution led to an echo train of 66 ms. Although the current implementation still lacks user-friendliness, 2DRF pulses are likely to become a useful addition to the arsenal of advanced MRI tools. .