Spatially 2D-selective RF excitations using the PROPELLER trajectory: basic principles and application to MR spectroscopy of irregularly shaped single voxel.

Spatially two-dimensional selective radio frequency (2DRF) excitations are able to excite arbitrarily-shaped profiles in their excitation plane and, hence, can be used to minimize partial volume effects in single-voxel magnetic resonance spectroscopy. In this study, 2DRF excitations based on the PROPELLER trajectory which consists of blades of parallel lines that are rotated against each other, are presented. Because the k-space center is covered with each segment, the trajectory yields a high signal efficiency which, e.g., is considerably improved compared to a segmented blipped-planar approach. It is shown that a sampling density correction based on the PROPELLER trajectory's Voronoi diagram suppresses unwanted side excitations. Off-resonance effects like chemical-shift displacement artifacts, can be minimized by applying nonselective refocusing radio frequency pulses between the lines of a blade. With half-Fourier segments, the 2DRF's echo time contribution can be shortened considerably. Thus, robust 2DRF excitations capable of exciting high-resolution profiles at short echo times with high signal efficiency are obtained. Their applicability to MR spectroscopy of an arbitrarily-shaped single voxel is demonstrated in a two-bottle phantom and in the human brain in vivo on a 3 T whole-body MR system.