The impact of respiratory motion on electromagnetic fields and specific absorption rate in cardiac imaging at 7T

To present electromagnetic simulation setups for detailed analyses of respiration's impact on $B_1^{+}$ and E-fields, local specific absorption rate (SAR) and associated safety-limits for 7T cardiac imaging.

Finite-difference time-domain electromagnetic field simulations were performed at five respiratory states using a breathing body model and a 16-element 7T body transceiver RF-coil array. $B_1^{+}$ and SAR are analyzed for fixed and moving coil configurations. SAR variations are investigated using phase/amplitude shimming considering (i) a local SAR-controlled mode (here SAR calculations consider RF amplitudes and phases) and (ii) a channel-wise power-controlled mode (SAR boundary calculation is independent of the channels' phases, only dependent on the channels' maximum amplitude).

Respiration-induced variations of both $B_1^{+}$ amplitude and phase are observed. The flip angle homogeneity depends on the respiratory state used for $B_1^{+}$ shimming; best results were achieved for shimming on inhale and exhale simultaneously ( ). The results reflect that respiration impacts position and amplitude of the local SAR maximum. With the local-SAR-control mode, a safety factor of up to 1.4 is needed to accommodate for respiratory variations while the power control mode appears respiration-robust when the coil moves with respiration (SAR peak decrease: 9% exhale→inhale). Instead, a spatially fixed coil setup yields higher SAR variations with respiration.

Respiratory motion does not only affect the $B_1^{+}$ distribution and hence the image contrast, but also location and magnitude of the peak spatial SAR. Therefore, respiration effects may need to be included in safety analyses of RF coils applied to the human thorax.