kT-points: Short three-dimensional tailored RF pulses for flip-angle homogenization over an extended volume

With Transmit SENSE, we demonstrate the feasibility of uniformly exciting a volume such as the human brain at 7T through the use of an original minimalist transmit k-space coverage, referred to as kT-points. Radio-frequency energy is deposited only at a limited number of k-space locations in the vicinity of the center to counteract transmit sensitivity inhomogeneities. The resulting nonselective pulses are short and need little energy compared to adiabatic or other B1+-robust pulses available in the literature, making them good candidates for short-repetition time 3D sequences at high field. Experimental verification was performed on three human volunteers at 7T by means of an 8-channel transmit array system. On average, whereas the standard circularly polarized excitation resulted in a 33%-flip angle spread (standard deviation over mean) throughout the brain, and a static radio-frequency shim showed flip angle variations of 17% and up, application of kT-point-based excitations demonstrated excellent flip angle uniformity (8%) for a small target flip angle and with sub-millisecond durations. Magn Reson Med, 2011. (C) 2011 Wiley-Liss, Inc.