BEST IN PHYSICS (IMAGING): Breaking the Frequency Limit: The First Quadrature RF Volume Head Coil for 10.5T MRI
Abstract
Purpose
To develop the first quadrature RF volume coil for 10.5T MRI, using metasurface technology to overcome limitations of traditional volume coil designs.
Methods
The metasurface unit cells consist of conductive crosses interlinked by capacitors, allowing control of propagation mode dispersion in a 310mm ⌀ circular waveguide. The design target is a mode with a homogeneous transverse H field at the proton Larmor frequency of 10.5T MRI (447MHz). The metasurface is on a cylinder of 280mm ⌀ for comparison with an existing 16 port parallel transmit (pTx) array. Initial capacitor values were determined using relatively quick eigenmode simulations in Ansys HFSS. Full structure simulations provided the electromagnetic fields in presence of a lightbulb-shaped head phantom (17cm maximum diameter, εr=47.26, σ=0.65S/m); co-circuit simulations (CoSimPy) were performed for final tuning. The coil was constructed by patterning the unit cells onto flexible PCBs (polyimide substrate), which were soldered together and wrapped around a polycarbonate former. Ports were installed at two orthogonal locations like a conventional birdcage coil. The transmit (B1+) efficiency was measured using the actual flip angle imaging (AFI) sequence for both coils driven in circular polarization mode in a 10.5T scanner.
Results
The simulated and measured B1+ efficiency of the metasurface coil were in good agreement, with peak values of 0.18μT/√W and 0.19μT/√W respectively. The pTx array had a similar field pattern but with a peak B1+ efficiency of 0.37μT/√W. The simulated safety efficiency of the metasurface coil was especially high (0.89μT/√(W/kg) compared to that of the pTx array (0.37μT/√(W/kg).
Conclusion
The metasurface coil provides remarkable transmit efficiency and homogeneity with the simplicity of a two-port coil. Its insensitivity to loads and minimal number of ports greatly simplifies the imaging workflow at 10.5T compared to pTx arrays which are complicated to fabricate and operate.