Optimized Outer Volume Suppression for Slice Selective MRSI

A. Henning, M. Schär, R. F. Schulte, K. P. Pruessmann, P. Boesiger Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Kanton Zurich, Switzerland, John Hopkins University, Baltimore, MD, United States Introduction In spite of significantly improved saturation pulses [1,2] sufficient outer volume (OVS) and especially fat suppression in MRSI could only be achieved in combination with PRESS localization [3]. PRESS causes immense chemical shift displacement at higher field strength. Therefore metabolite ratios in spectra at outer regions of the FOV are not reliable. Over-prescribed PRESS MRSI, in which the non-overlapping areas of the excited volumes are saturated by OVS bands, was only shown in combination with single suppression bands [3]. Slice selective MRSI, which guarantees correct metabolite ratios, has been unfavorable due to high lipid content in the spectra. In this work the flip angles of multiple, overlapping suppression bands were optimized considering crossing and progressing T1 relaxation during the OVS scheme. Therefore sufficient, T1 and B1 insensitive outer volume suppression in combination with over-prescribed PRESS MRSI and even slice selective MRSI could be achieved. The optimization was performed for higher-order phase pulses (HOPP), which combine a high selectivity with in comparison to VSS [1,3] or QPP [2,4,5] an even larger bandwidth and hence a negligible chemical shift displacement. Materials and Methods The flip angle optimization was based on Bloch equations. In a first step a simplified model, neglecting bandwidth and profile of the HOPP pulses and the performance of spoiling gradients, has been chosen to calculate the optimal flip angles for each saturation band. For a single suppression band of the total duration τ, consisting of a RF-pulse with the flip angle φ, a slice selective and a spoiling gradient and a delay, the residual longitudinal magnetization MR can be calculated from ) 1 ( cos 1 1 / / T O T I R e M e M M τ τ φ − − − + = . MO is the equilibrium magnetization; MI is the magnetization at the beginning of the