RF B1 field localization through convex optimization



A 9-cm wavelength in the human brain at the 400-MHz Larmor frequency for 9.4-T leads to pronounced B1 field contours and consequential image inhomogeneity. The objective of this study was to develop a new technique to control this nonuniformity by designing the phase and magnitude of radiofrequency (RF) power emanating from RF coil antenna elements in a multichannel transceiver array. Nonuniformity was used to steer a constructively interfering B1 field node to spatially correlate with an anatomic region of interest. This work outlines a convex (quadratically constrained quadratic problem) formulation of the B1 localization problem and the benefits of such a formulation. This convex formulation was used to design antenna excitation magnitude and phase. Localization is demonstrated in simulated finite difference time domain (FDTD) B1 field human head distributions and human head phantom measurement. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:31–37, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26456