The dual-mode dipole: A new array element for 7T body imaging with reduced SAR

Abstract

Purpose: To design and test an RF-coil based on two orthogonal eigenmodes in a pair of coupled dipoles, for 7 Tesla body imaging with improved SAR, called dual-mode dipole.

Methods: The proposed coil consists of two dipoles and creates two orthogonal field distributions in a sample (the even and odd modes). A coupler used to excite the modes was miniaturized with the conductor track routing technique. Numerical simulations of the dual-mode dipole in the presence of a homogeneous phantom were performed. Moreover, an array of such coils was simulated with a voxel body model. For comparison, a fractionated dipole combined with a surface loop coil was also simulated. Both coils were tested in a 7 Tesla MRI system on a phantom. Subsequently four dual-mode dipoles or dipole/loop combinations were used for a comparison of imaging performance in a human volunteer.

Results: Using the even mode of the dual-mode dipole showed 70% SAR reduction in comparison to the fractionated dipole while having the same $B_1^{+}$ in the prostate region. The odd mode of the dual-mode dipole showed a performance comparable to the surface loop both for SAR and B₁ efficiency. The obtained results showed that the proposed coil while creating lower SAR gave images of the same quality as the reference coil.

Conclusions: It was demonstrated that the array of dual-mode dipoles provided the same SNR and prostate imaging quality as the reference array, while demonstrating lower SAR. This is due to a smoother current distribution over a sample surface.

Keywords: Dipole; SAR; coil; modes; parallel transmit.