Regions of constructive and destructive interference of the B1+ field occur in high-field MRI as the RF wavelength is of the same order as the imaging region. Several techniques exist for B1+ shimming including the use of transmit arrays and k-space RF pulse design. “Dielectric pads” using high permittivity materials have been shown to be a simple and widely applicable alternative, particularly in neuroimaging at 7T 1-3. The aims of this current work are to improve the practical application of such pads in three different areas: (i) to provide a simple method to design pads with a specific permittivity, (ii) to improve the properties of the dielectric pad in terms of rigidity and long-term stability, and (iii) to test whether integration of devices such as headphones can be achieved while maintaining the advantages of the improved B1+ homogeneity.Mixtures with different barium titanate (εr ≈ 2000) and calcium titanate (εr ≈ 160) ratios in deionized water were prepared . Complex permittivity measurements were performed using a SPEAG Dielectric Assessment Kit DAK-12 . Data were fitted to different mixing rules to determine the one which best fits the tri-component properties. The experimental measurements were repeated with different gelling agents (agar, xantham gum, phytagel and hydroxyethyl-cellulose (HEC)) added to increase the viscosity of the dielectric pad. To determine the effect of integrating headphones into the dielectric pad (by introducing a hole at the level of the ears), electromagnetic simulations were performed using xFDTD (7.4.0.2, Remcom, PA, United States) and the Duke body model . Based on the simulation results, dielectric pads were constructed using a mixture of calcium and barium titanates to give a relative permittivity of 150 with hole dimensions of 65 x 55mm. B1+ maps of the brain were obtained using the DREAM sequence⁴, as well as anatomical turbo spin echo images, on a 7T Achieva MRI system (Philips Achieva, Philips Healthcare, Best, the Netherlands) in healthy volunteers.

The most appropriate mixing rule for the permittivity of aqueous suspensions of mixtures of barium titanate and calcium titanate was determined to be the Lichtenecker logarithmic law⁵

ε_eff=ε_Ca^fCa ε_Ba^fBa ε_w^fw

where f represents volume fractions of calcium titanate, barium titanate and water. Figure 1 shows the fitted plot to individual data points, allowing the design a pad with any permittivity between 110 and 300. Of the different gelling agents used, the stability (both short-term and long-term) of the aqueous dielectric suspensions was improved most using 0.5 g of HEC per 100 ml of water. The addition of HEC to the dielectric material has no impact on the permittivity of the suspension but increases the conductivity from 0.1 S/m to 0.15 S/m. EM simulations showed that the impact of this increased conductivity on the performance of the pad in vivo is minimal. The results of the EM simulations to study the effect of a hole in the dielectric pads are shown in Figures 2 and 3, in terms of the B1+ and displacement currents in the pads, respectively. For a fixed permittivity of 110 a drop in the secondary B1+ created by the pad is seen at the location of the hole (Figure 2), but there is still significant secondary field enhancement elsewhere. Increasing the permittivity of the dielectric material in the pad to 150 largely compensates for the drop in the B1+ caused by the hole. Figure 3 shows that the displacement currents within the pad are not severely affected (except at the location of the hole) explaining the improved performance (compared to no pad) even in the presence of the hole. Figure 4 shows in vivo results using a turbo spin echo imaging sequence. In the absence of dielectric pads there is a well-characterized severe loss in image contrast and signal intensity in the temporal lobe and cerebellum. Using the dielectric pads with a hole for the earpads/headphones (shown in Figure 5) and a relative permittivity of 150 gives substantial improvements in image quality, similar to the effects of a full pad with relative permittivity of 110.

Using the Lichteneker logarithmic mixing rule allows a range of permittivities to be used for the construction of dielectric pads. Hydroxyethyl cellulose can be used to improve the stability of the dielectric suspensions with minimal impact on the efficacy of the dielectric pad. Introduction of a hole in the pad to accommodate headphones (or other devices) leads to a drop in the secondary B1+ produced by the pad at the location of the hole, but increasing the permittivity of the pad to ~150 produces very similar increases in image uniformity as the conventional full pad with relative permittivity 110.