The installation of the world first 10.5T human MRI system at the University of Minnesota, the Center for Magnetic Resonance Research (CMRR) once again, pushes the boundary of human MRI technology into an uncharted territory. With the increased field strength, prominent wave behavior exhibited at 7T is expected to be even stronger, which could adversely impact anticipated SNR benefits and (local and global) SAR limitations at the higher field strength. Thus, in order to seize the full potential of this unique instrument for human brain research, it is necessary to first carefully examine the RF behavior in the human head at this new field strength. The wave behavior seen at high fields stems from the dielectric properties of the human body, which lead to the wave propagation patterns at 300 and 447 Mhz. In fact, the dielectric effect originally inspired the use of high dielectric constant (HDC) materials to manipulate the wave behavior in an attempt to increase SNR and decrease SAR with monolithic high dielectric constant materials as has been previously shown at lower frequencies^1-3. This study characterized RF wave behavior at 10.5T in human brain model and phantom in comparison to that at 7T and explored the potential for even greater improvement of SNR using high dielectric materials at 10.5T.

Figure 1a shows the baseline computer modeling setup, consisting of a human head model (Virtual Family Ella) with an 8 cm diameter T/R surface coil driven by unit current sources. The simulation was performed at 300 MHz for 7T and 447 MHz for 10.5T using xFDTD (REMCOM, Inc., PA). At 10.5T a parameter sweep for a dielectric disk (placed between the head and the coil as shown in Fig. 1b) was run to determine an approximate starting permittivity for this coil within our manufacturing constraints. A disk (diameter 80 mm, thickness 10 mm) with a permittivity of 90 was selected as it both strongly and homogeneously enhanced B₁⁺ up to our desired depth of about 5 cm. This disk was then manufactured, characterized and finally used for the experimental part of the study. Additionally, we constructed an RF coil (ID 80 mm), which was placed in a custom coil holder (drawer like for coil height adjustment, see Fig. 1c). For the baseline the coil was as close as possible to the phantom (8.2g Na C₂H₃O₂ + 9.6g C₃H₅O₃Li per L). This phantom was placed on top of the setup as shown in Fig. 1d. Once the dielectric was placed between coil and phantom the coil was moved about 1 cm away from the phantom to make room for the disk. Capacitor values for tuning and matching were adjusted for each case. Experimentally, relative B₁⁺ (AFI technique⁴), receive sensitivity and noise scans were obtained on the Siemens Magnetom 10.5T MR-system. Flip angle and RF power were carefully adjusted to compare baselines and HDC cases.

Our experimental and simulated results demonstrate that stronger wave behaviors associated with the dielectric effect at 10.5T could make RF shimming more effective than at 7.0T. Incorporation of high dielectric material into the RF coil could synergistically improve SNR and RF shimming by reducing the overall necessary RF power to achieve the same flip angle, thus reducing the overall and local SAR⁵ constrains during RF shimming.