Stacked magnetic resonators for MRI RF coils decoupling

Design of Distributed Spiral Resonators for the Decoupling of MRI Double-Tuned RF Coils

OBJECTIVE

A systematic analytical approach to design Spiral Resonators (SRs), acting as distributed magnetic traps (DMTs), for the decoupling of concentric Double-Tuned (DT) RF coils suitable for Ultra-High Field (7 T) MRI is presented.

METHODS

The design is based on small planar SRs placed in between the two RF loops (used for signal detection of the two nuclei of interest). We developed a general framework based on a fully analytical approach to estimate the mutual coupling between the RF coils and to provide design guidelines for the geometry and number of SRs to be employed. Starting from the full-analytical estimations of the SRs geometry, electromagnetic simulations for improving and validating the performance can be carried out.

RESULTS AND CONCLUSION

We applied the method to a test case of a DT RF coil consisting of two concentric and coplanar loops used for 7 T MRI, tuned at the Larmor frequencies of the proton (¹H, 298 MHz) and sodium (²³Na, 79 MHz) nuclei, respectively. We performed numerical simulations and experimental measurements on fabricated prototypes, which both demonstrated the effectiveness of the proposed design procedure.

SIGNIFICANCE

The decoupling is achieved by printing the SRs on the same dielectric substrate of the RF coils thus allowing a drastic simplification of the fabrication procedure. It is worth noting that there are no physical connections between the decoupling SRs and the ¹H/²³Na RF coils, thus providing a mechanically robust experimental set-up, and improving the transceiver design with respect to other traditional decoupling techniques.

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

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.

Metamaterial Combining Electric- and Magnetic-Dipole-Based Configurations for Unique Dual-Band Signal Enhancement in Ultrahigh-Field Magnetic Resonance Imaging

Magnetic resonance imaging and spectroscopy (MRI and MRS) are both widely used techniques in medical diagnostics and research. One of the major thrusts in recent years has been the introduction of ultrahigh-field magnets in order to boost the sensitivity. Several MRI studies have examined further potential improvements in sensitivity using metamaterials, focusing on single frequency applications. However, metamaterials have yet to reach a level that is practical for routine MRI use. In this work, we explore a new metamaterial implementation for MRI, a dual-nuclei resonant structure, which can be used for both proton and heteronuclear magnetic resonance. Our approach combines two configurations, one based on a set of electric dipoles for the low frequency band, and the second based on a set of magnetic dipoles for the high frequency band. We focus on the implementation of a dual-nuclei metamaterial for phosphorus and proton imaging and spectroscopy at an ultrahigh-field strength of 7 T. In vivo scans using this flexible and compact structure show that it locally enhances both the phosphorus and proton transmit and receive sensitivities.