Investigation of Metamaterial Transmission line resonators for Ultra-High Field Magnetic Resonance Imaging RF Coils

Abstract

The objective is to develop a highly efficient RF head coil on a thin substrate for the Ultra-high magnetic field (7 T and above) MRI systems. The artificial Metamaterial resonator is investigated for this purpose. Simulation and experimental results are provided for an 8-channel Metamaterial based RF coil in comparison with a standard high performance 8-channel dipole based RF coil for the 10.5 T MRI system. Each element is 180 mm (approximately a quarter of a wavelength λ0) long, identical, evenly spaced along the circumference of the cylindrical phantom, loaded with dielectric material, and referred to as inverted Metamaterial Zeroth Order Resonator. The resonator elements are open circuited, matched, and tuned to 447.06 MHz with the phantom. An unloaded to loaded quality factor ratio of 2.97 is obtained from the scattering matrix of the proposed design. The length independent nature of the proposed design and the flexibility of the lumped elements have provided an optimized element with a substrate thickness of roughly 3 mm (λ0/200). With the proposed design, a RF magnetic field strength (B1+) to √SAR ratio of 1.38 (compared to 1.46 for dipole) is obtained. Optimization of the physical design parameters, especially the distance between the element and the phantom, is performed to improve the transmission efficiency of the metamaterial based RF coil element. The amount of radiated power reaching 45 mm inside the phantom is used for the comparison. The optimal design for a 16 cm long, 7 T metamaterial resonator shows an increase of 1.1 dB and 3.2 dB in transmit power when compared to a dipole and microstrip element of same length. Similarly, the optimal design for an 18 cm long, 10.5 T metamaterial resonator shows an increase of 1.6 dB in power when compared to a 12 cm long microstrip and a 0.2 dB decrease in power when compared to an 18 cm long dipole. An electron band-gap (EBG) periodic structure is designed as the ground plane for the proposed metamaterial resonator. The simulation results show increased B1+ field magnitude when compared with the metamaterial resonator with a solid ground plane. Similarly, a technique of improving the surface current density by creating slots along the line is implemented in the 7 T microstrip resonator. The experimental results show an improved coil efficiency after including of the slots. The extendibility of the coil conductor length with the Metamaterial resonators is also shown by designing a 48 cm long, 10.5 T metamaterial loop body element (longer than a wavelength). Simulations and experimental results confirm the functionality of the loop element and show the comparison with a traditional loop element.