The concept of traveling-wave MRI has been introduced¹ for ultra-high fields, enabling large field-of-view (FOV) excitation and physical separation between the antenna and the subject. However, the efficiency of such an RF transmit setup is usually much lower than conventional methods due to the large excitation coverage^1,2. Several studies have shown that introducing additional materials/structures into the magnet bore or surrounding the subject can alter the RF field propagation in order to improve the efficiency in the region of interest. These include adding a dielectric lining³ placed between the antenna and the subject to reduce the attenuation, as well as a recent study with metamaterials placed close to the head⁴. In this current work we explore the use of high permittivity material placed behind (with respect to the direction of traveling wave propagation) the region of interest and show that it can be beneficial for traveling wave efficiency due to an electromagnetic “refraction” effect from the added dielectric material. Separating the region of improved efficiency from that of the dielectric allow positioning of a receive array in the close proximity to the region of interest, physically separate from the dielectric material. 3D Electromagnetic simulations were performed to calculate the B₁⁺ efficiency in terms of the relative permittivity (ε_r) and material dimensions using finite integration technique (FIT) software (CST Microwave Studio, Darmstadt, Germany). All B₁⁺ maps were normalized to an accepted power of 1 Watt. Two setups were examined in simulations: i) the brain as the region of interest with two-halves of an annular cylinder of dielectric material positioned around the neck, and ii) the knee as the region of interest with an annular cylinder of dielectric material placed around the thigh. In vivo experiments in the brain were performed using a quadrature segmented dipole antenna⁵ for both transmit and receive (see Figure 1). Data were acquired to compare images with and without two halves of an annular cylinder filled with water with dimensions of 100 mm outer radius, 60 mm inner radius and 80 mm height. MRI experiments were performed on a Philips Achieva 7 T MRI system. A gradient echo sequence was run with the following parameters: FOV 28 x 28 cm², spatial resolution 1.5 x 1.5 x 7.0 mm³, TR/TE of 700/2.72 ms.Figure 1 shows simulation results for the B₁⁺ variation in the brain for a set of dielectric material permittivity values and outer radii. There was no change in the maximum 10g SAR value when the cylinder was in place, but an average increase of ~20% in the transmit efficiency, with a maximum value of ~50%. Increasing the radius of the dielectric material improves the efficiency of the B₁⁺ in the brain. However, an increased permittivity has a non-linear effect, reaching a maximum in the range of ε_r equal to 120. Figure 2 shows the results from in-vivo scanning of the brain of a volunteer with the two-halves of the annular cylinder placed around the neck as described above. Figure 3 shows a comparison of the B₁⁺ maps with and without the dielectric material for imaging of the knee. The average increase of B₁⁺ obtained in the region of interest was 20%, with a local maximum increase of 67%; the maximum SAR (10 g spatially averaged) was increased by 27%, so overall there is a minor increase in the B₁⁺ per square root of maximum SAR. This is not surprising since the leg forms an ideal shape for travelling wave MRI¹, effectively forming a tapered structure which does not produce large reflections, unlike the case in the brain². Figure 3 also shows an effect which can be thought of as electromagnetic refraction, in which the RF bends towards the high permittivity material, producing higher efficiency in front of the dielectric.By placing high permittivity material at a location physically separate from the imaging region of interest, the transmit efficiency can be improved directly “in front” of the material, allowing close fitting receive arrays to be used for high sensitivity MR detection. Electromagnetic simulations and in vivo results show, on average, a 20% gain in efficiency using this approach.