Introduction:

One way to address the B₁⁺ inhomogeneity in ultra-high field MRI (B₀≥7T) is the use of relative High-Dielectric Constant (HDC) materials in radiofrequency (RF) coils [1]. Their high displacement currents alter the global RF distribution in the transmit coil and generate a secondary localized RF field used to tune the B₁⁺ field distribution. Most current effective HDCs are currently made into small rectangular pads filled with BaTiO₃ water mixtures. Unfortunately, major drawbacks prevent their use in clinical routine: drops in performance over time [1] and toxicity during preparation [2]. A new dielectric material combined with an innovative solvent is presented here to mimic BaTiO₃ pads performance while addressing those issues to pave the way for new long-lasting, comfortable and efficient pads.

Materials and Methods:

Silicon Carbide (SiC) spherical particles (Neyco, Vanves, France) were mixed with 4-Fluoro 1.3-dioxalan-2-one (Chemos, Regenstauf, Germany), also called FEC, with powder volume ratio contents ranging from 5% to 60%. Particles average size is around 2 µm to ensure good particle density in the composite material. Volume contents of Polyethylene glycol (Sigma-Aldrich, Saint Louis, USA), also called PEG, is added as a dispersant agent for the slurry homogeneity. Its quantity is fixed to 10% for volume contents of SiC below 50% and to 1% above 50% to ensure high permittivity levels in the sample. Samples above 50% are machine-pressed. Its number average molar mass is 10 000. Permittivity measurements on the mixtures have been carried out through Nicolson and Ross protocol [3] combined with a de-embedding process using an innovative coaxial cell and sample holder [4]. Rectangular 10x10x1 cm³ pads in polylactic-acid were filled with those mixtures in order to compare their performance to BaTiO₃ water pads. B₁⁺ maps were carried on using XFL sequences [5] with and without pads, using a birdcage head coil 1Tx/1Rx (Invivo Corp., Gainesville, USA) and a 3-D printed Specific Anthropomorphic Mannequin phantom (SAM; ε’=42; ε’’=60) on a 7T Magnetom MRI scanner (Siemens Healthineer, Erlangen, Germany).

Results:

Figure 1 shows permittivity levels reached by our composite materials at 300 MHz from 0 to 60% volume contents of SiC powder. Permittivity levels of BaTiO₃ water mixtures are also presented (from [6]). The effects of one SiC pad (ε’=288, ε’’=80) is compared to one pad filled with BaTiO₃ (ε’=241, ε’’=29) mixture. Its permittivity levels are chosen to have a similar effect to the BaTiO₃ pad given permittivity. To that end, B₁⁺ field distributions are displayed in Figure 2. In both case a B₁⁺ illumination is observed and the dark area located in the left temporal lobe is tightened in the vicinity of the pad.

Discussion:

Compared to BaTiO₃, a pad filled with SiC particles exhibits a lower B₁⁺ illumination owing to its higher conductivity. This is a consistent issue when using dispersant agents [1]. But its penetrating effect can be tuned by adjusting the pad permittivity from 100 to 300. Thus, this new HDC material displays the required permittivity range to be effective for MR imaging at 7T. It also has the potential for being daily used in clinical routines. First, it overcomes the main limitation of the current water based pads which is its decrease in performance over time [1]. Indeed, the combination of its solvent and dispersant agent allows a long lifespan: permittivity levels were preserved after 6 months. Since the solvent fusion temperature is close to 25°C, the mixture can be stored at lower temperature to prevent particles sedimentation. Pads are also less prone to dry out as the solvent boiling point is twice the water boiling temperature. Then, the overall pasty consistence allows the design of comfortable pads that are not likely to break and deform when used on a daily basis. Finally, toxicity threat is much lower than BaTiO₃ powders which makes it easier, safer and faster to prepare.

Acknowledgements

This work has been supported by the Programme Transversal du CEA and the FET-OPEN M-CUBE Project.