Rapid fat suppression for three-dimensional echo planar imaging with minimized specific absorption rate.

Abstract

To investigate a method for rapid water excitation with minimal radiofrequency power deposition for efficient functional MRI at ultrahigh fields. The suitability of the spectral response of a single rectangular radiofrequency pulse (rect) as a replacement of conventional fat saturation in segmented three-dimensional (3D) echo planar imaging (EPI) is explored. A pulse duration formula for lipid signal nulling independent of the small-tip-angle approximation is derived and tested by means of simulations and experiments at 3 and 7 Tesla (T). Compared with conventional binomial-11 water excitation, the single rect method is more selective and less sensitive to shim imperfections. In functional MRI, a significant measurement speedup (25%) and specific absorption rate reduction (from 44% to 1% at 7T) compared with conventional fat saturation are achieved. Furthermore, magnetization transfer effects are reduced resulting in up to 25% higher brain tissue signal-to-noise ratio. The proposed method is well suited for whole-brain functional MRI, not only at ultra-high fields, as it maximizes the sensitivity per unit time and at the same time minimizes radiofrequency power deposition. It requires little implementation effort and may thus be used in other spatially nonselective imaging methods that require fat suppression at minimal specific absorption rate and time requirements. Magn Reson Med 76:1517-1523, 2016. © 2015 International Society for Magnetic Resonance in Medicine.