Dynamically applied B₁⁺ shimming solutions for non-contrast enhanced renal angiography at 7.0 tesla

The purpose of this study was to detail a strategy for performing non-contrast enhanced renal magnetic resonance angiography studies at 7.0 T. It is demonstrated that with proper B 1+ management, these studies can be successfully performed at ultrahigh field within local specific absorption rate constraints. An inversion prepared gradient echo acquisition, standard for non-contrast renal magnetic resonance angiography studies, required radiofrequency pulse specific B 1+ shimming solutions to be dynamically applied to address the field dependent increases in both B₀ and B 1+ inhomogeneity as well as to accommodate limitation in available power. By using more efficient B 1+ shimming solutions for the inversion preparation and more homogeneous solutions for the excitation, high quality images of the renal arteries were obtained without venous and background signal artifacts while working within hardware and safety constraints. Finite difference time domain simulations confirmed in vivo measurements with respect to B 1+ distributions and homogeneity for the range of shimming strategies used and allowed the calculation of peak local specific absorption rate values normalized by input power and B 1+. Increasing B 1+ homogeneity was accompanied by decreasing local specific absorption rate per Watt and increasing maximum local specific absorption rate per [B 1+]², which must be considered, along with body size and respiratory rate, when finalizing acquisition parameters for a given individual. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.