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

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.

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.

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.

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.