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 “k_T‐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 B‐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 k_T‐point‐based excitations demonstrated excellent flip angle uniformity (8%) for a small target flip angle and with sub‐millisecond durations. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.