Synopsis

Traveling-wave MRI has robust matching performance and capability for large field-of-view (FOV) imaging. However, the efficiency of traveling-wave MRI is much lower than conventional methods, which limits its application. One way to improve the efficiency is to place local wireless resonators around the subject. The feasibility of this approach has been demonstrated in previous works using a single small loop. However, it is not clear whether other kinds of coils (such as electric dipoles) can be used as local elements, and it is not clear how much the improvements can be maintained in human imaging using an array design. By using wireless local loop coil and transverse dipole arrays, the transmit efficiency (B₁⁺) of traveling-wave MRI can be improved by 3.4-fold in the brain and 2-fold in the knee. The coil types (loops or dipoles) should be carefully chosen for brain or knee imaging to maximize the improvement since they exhibit different types of coupling to the TE₁₁ mode, and the enhancement depends on the local body configuration

Introduction

Methods

Fig. 1A illustrates a traveling wave MRI experiment with wireless local coils. The wireless coil produces a secondary magnetic field H’, which locally enhances B₁ efficiency. To explore whether electric dipoles can locally enhance the B₁ field, traveling-wave MRI was simulated in an empty bore with a local longitudinal (along the z-direction) dipole and a transverse dipole. Simulation results are shown in Figs. 1B-1D. It is found that the longitudinal dipole has no influence on the original B₁⁺ field, but the transverse dipole can improve B₁⁺ in front of it, which is expected since only the transverse dipole is coupled to the TE₁₁ mode and acts as a reflector. Four local array setups were then examined in human body simulations: (I) brain with an 8-channel wireless loop array (Fig. 2A); (II) knee with an 8-channel wireless loop array (Fig. 2B); (III) brain with a 4-channel transverse dipole array (Fig. 2C); (Ⅳ) knee with a 4-channel transverse dipole array (Fig. 2D). Inductor-segmented crossed-dipoles were used as the traveling-wave antenna. Traditional traveling-wave setups without wireless coils were also simulated as comparisons.

In all simulations, the total input power was set to 2 Watts. Simulations were performed using finite-element-method (Ansys HFSS, Canonsburg, PA, USA). To validate the simulation results, MR experiments were performed on a 7T whole-body scanner (Philips Healthcare, Best, Netherlands). GRE images and B₁⁺ maps were acquired on a head-shaped phantom using the traveling-wave antenna with and without an 8-channel wireless local loop array. The parameters of the GRE sequence were: FOV=20x20 cm², spatial resolution 2x2x5 mm³, TR/TE=1000/2.7 ms. B₁⁺ maps were measured using DREAM method with the same input power⁸.

Results

Fig. 3 shows the simulated B₁⁺ efficiency in the brain. The wireless loop array increased the average B₁⁺ field strength 3.4-fold. In the brain, most of the traveling wave transmit power is reflected by shoulders and wasted. With local loops, this reflection can be avoided since the most power deposits in the loops before arriving at the shoulders. For the wireless dipole array, however, the B₁ efficiency is actually slightly lower than with traveling-wave alone, since the dipole array enhances this reflection effect.

Fig. 4 shows the simulated B₁⁺ efficiency in the knee. The local loop array resulted in a 1.5-fold increase in average B₁⁺ field strength. Transmit power must travel across conductive tissues such as feet and legs before the knees. Therefore the improvement is relatively lower compared to the brain since much of the power is deposited before arriving at the knees. In comparison, the local dipole array increased the average B₁⁺ field strength 2-fold, which is different from that in brain imaging. This is due to the large distance between the traveling wave antenna and the local dipole array, in which case the input RF signal and the reflected RF signal are deposited locally (red arrows in Fig. 4C).

Fig. 5 shows the traveling wave experimental results on a head-shaped phantom with and without an 8-channel loop array. As expected from the simulation results (Fig. 3B vs Fig. 3A), the B₁⁺ efficiency increased approximately 3.5-fold using the local loop array.

Conclusion

Acknowledgements

References

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