Improved RF performance of travelling wave MR with a high permittivity dielectric lining of the bore

Abstract

Application of travelling wave MR to human body imaging is restricted by the limited peak power of the available RF amplifiers. Nevertheless, travelling wave MR advantages like a large field of view excitation and distant location of transmit elements would be desirable for whole body MRI. In this work, improvement of the B1+ efficiency of travelling wave MR is demonstrated. High permittivity dielectric lining placed next to the scanner bore wall effectively reduces attenuation of the travelling wave in the longitudinal direction and at the same time directs the radial power flow toward the load. First, this is shown with an analytical model of a metallic cylindrical waveguide with the dielectric lining next to the wall and loaded with a cylindrical phantom. Simulations and experiments also reveal an increase of B1+ efficiency in the center of the bore for travelling wave MR with a dielectric lining. Phantom experiments show up to a 2-fold gain in B1+ with the dielectric lining. This corresponds to a 4-fold increase in power efficiency of travelling wave MR. In vivo experiments demonstrate an 8-fold signal-to-noise ratio gain with the dielectric lining. Overall, it is shown that dielectric lining is a constructive method to improve efficacy of travelling wave MR.

Improved traveling-wave efficiency in 7 T human MRI using passive local loop and dipole arrays

“…One way to improve the transmit efficiency is to deliver traveling-wave power directly to the target imaging region with a coaxial waveguide [10]. Another method is to introduce additional materials into the magnet bore, such as metamaterials [11] and dielectric fillings with specific geometry and permittivity [12,13]. An alternative simple method proposed recently is to place local resonators around the subject [14-16].…”

Improved steering of the RF field of traveling wave MR with a multimode, coaxial waveguide

“…The dielectric lining was present in both feeding and imaging sections of the setup. In our previous work , we showed that such a water tube array can augment RF incoupling efficiency. Here, we demonstrate that the water tube array is also advantageous for RF shimming.…”

“…However, only propagating modes (or evanescent modes which are close to their cut‐offs) will reach the imaging section of the multimode, coaxial waveguide. To increase the amount of such modes, the cut‐off frequencies of the higher order modes were reduced by increasing the effective dielectric permittivity of the MR bore by a high permittivity dielectric lining adjacent to the wall of the bore consisting of densely packed water tubes . The dielectric lining was present in both feeding and imaging sections of the setup.…”

Quadrature operation of segmented dielectric resonators facilitated with metallic connectors

“…Ultrahigh field [7 Tesla (T) and above] human MRI provides the impetus for new radiofrequency coil designs. Thus, in addition to modifications to existing designs (1) such as the birdcage coil (2), transverse electromagnetic mode resonator (3) and surface coil, new concepts such as travelling wave antennas (4)(5)(6)(7)(8), dipoles (9)(10)(11)(12), cavity resonators (13) and dielectric resonators (DRs) (14,15) have been shown, both for single coils as well as for transmit arrays. Transmit arrays (16)(17)(18) offer a high degree of flexibility in terms of shaping the transmit magnetic (B þ 1 ) field and are essential for body imaging at very high fields.…”