Whole body traveling wave magnetic resonance imaging at high field strength: Homogeneity, efficiency, and energy deposition as compared with traditional excitation mechanisms

In 7 T traveling wave imaging, waveguide modes supported by the scanner radiofrequency shield are used to excite an MR signal in samples or tissue which may be several meters away from the antenna used to drive radiofrequency power into the system. To explore the potential merits of traveling wave excitation for whole-body imaging at 7 T, we compare numerical simulations of traveling wave and TEM systems, and juxtapose full-wave electrodynamic simulations using a human body model with in vivo human traveling wave imaging at multiple stations covering the entire body. The simulated and in vivo traveling wave results correspond well, with strong signal at the periphery of the body and weak signal deep in the torso. These numerical results also illustrate the complicated wave behavior that emerges when a body is present. The TEM resonator simulation allowed comparison of traveling wave excitation with standard quadrature excitation, showing that while the traveling wave B+1 per unit drive voltage is much less than that of the TEM system, the square of the average B+1 compared to peak specific absorption rate (SAR) values can be comparable in certain imaging planes. Both systems produce highly inhomogeneous excitation of MR signal in the torso, suggesting that B₁ shimming or other parallel transmission methods are necessary for 7 T whole body imaging.