RF Shimming and Improved SAR Safety for MRI at 7 T With Combined Eight-Element Stepped Impedance Resonators and Traveling-Wave Antenna

A remote transmission and detection for magnetic resonance imaging (MRI) at 7 T was validated by using a patch antenna that excites the traveling-wave (TW) modes guided by the radio frequency (RF) shield of the MRI system. In this paper, RF simulations were performed at 298 MHz for an eight-element stepped impedance resonators (SIRs) head volume coil combined with a patch antenna. The combined structure is loaded with both a cylindrical phantom and the Duke full-body human voxel model. An optimization routine was developed in MATLAB to provide homogenous B-1(+) field distributions with minimal 10 g averaged specific absorption rate (SAR) values within the phantom and the Duke human biological model. Before optimization, the TW approach achieved a more homogenous B-1(+) field distribution providing a large field of view along the propagation direction compared to the SIRs head volume coil. However, the corresponding peak local SAR values for the TW approach is about 2.4 times higher than those of the SIRs head volume coil. The B-1(+) transmission efficiency for the TW approach is higher than that of the SIRs head volume coil by 10-30% due to the coupling of the TW modes into the volume coil. The RF-shimming technique improves the B-1(+) field homogeneity for the combined approach by 42%, 48%, and 47% in the central axial, sagittal, and coronal slices, respectively, and satisfies the constraints of maximum local SAR in the human head for MRI at 7 T.