Segmented 2D-selective RF excitations with weighted averaging and flip angle adaptation for MR spectroscopy of irregularly shaped voxel

Abstract

A weighted averaging scheme with flip angle adaptation is presented for segmented 2D-selective RF excitations and applied to single-line segments of a blipped-planar trajectory. Segments covering the central k-space, i.e. those with significant signal contributions, are averaged more often than the (outer) segments with low RF amplitudes and minor signal contributions. For compensation, these outer segments are applied with an increased RF amplitude, i.e. a larger flip angle, such that an unaltered signal contribution is obtained in a reduced number of shots. Numerical simulations and experiments in phantoms and the human brain in vivo demonstrate that the approach considerably increases the signal efficiency, i.e. the signal accumulated per time unit, without introducing profile distortions. Its application to single-voxel MR spectroscopy of a corpus-callosum-shaped region-of-interest yielded, due to an optimum coverage of the target volume, higher signal amplitudes than conventional localization based on cross-sectional RF excitations. Thus, the approach could improve the reliability of single-voxel MR spectroscopy.