Local SAR assessment for multitransmit systems: a study on the peak local SAR value as a function of magnetic field strength

With increasing permanent magnetic field strength B₀, the frequency of the radiofrequency field B₁ increases. The concomitantly reduced wavelength results in interference patterns that cause signal inhomogeneities and for ultrahigh field strengths even signal voids. These inhomogeneities are addressed by multitransmit systems. These systems use multiple transmit elements where the phase and amplitude of each element can be controlled to steer the B₁-field. However, in this way the electric field that originates from the B₁-field also changes. This may pose a safety risk in terms of localized tissue heating. To avoid potential hotspots, the local specific absorption rate (SAR) distribution needs to be determined by simulations. All concepts and methods to process simulation results are presented in this article. In addition, we use the presented methodology to investigate the relationship between the peak local SAR value and the magnetic field strength B₀. For this purpose, we have simulated the birdcage body coil at 3 T and appropriately designed 16-channel dipole-loop transceiver arrays at 3, 7, 10.5, and 14 T. We demonstrate a linear increase of peak local SAR with B₀ and a quadratic increase of signal-to-noise-ratio (SNR) with B₀.