Broadband Slab Selection with B1+ Mitigation at 7T via Parallel Spectral-Spatial Excitation

Chemical shift imaging benefits from signal-to-noise ratio (SNR) and chemical shift dispersion increases at stronger main field such as 7 Tesla, but the associated shorter radiofrequency (RF) wavelengths encountered require B-1(+) mitigation over both the spatial field of view (FOV) and a specified spectral bandwidth. The bandwidth constraint presents a challenge for previously proposed spatially tailored B-1(+) mitigation methods, which are based on a type of echovolumnar trajectory referred to as "spokes" or "fast-k(z)". Although such pulses, In conjunction with parallel excitation methodology, can efficiently mitigate large B-1(+) inhomogeneities and achieve relatively short pulse durations with slice-selective excitations, they exhibit a narrow-band off-resonance response and may not be suitable for applications that require B-1(+) mitigation over a large spectral bandwidth. This work outlines a design method for a general parallel spectra I-spatia I excitation that achieves a target-error minimization simultaneously over a bandwidth of frequencies and a specified spatial-domain. The technique is demonstrated for slab-selective excitation with in-plane B-1(+) mitigation over a 600-Hz bandwidth. The pulse design method is validated in a water phantom at 7T using an eight-channel transmit array system. The results show significant increases in the pulse's spectral bandwidth, with no additional pulse duration penalty and only a minor tradeoff in spatial B-1(+) mitigation compared to the standard spoke-based parallel RF design. Magn Reson Med 61:493-500, 2009. (C) 2009 Wiley-Liss, Inc.