Metasurface Approach to Generate Homogeneous B1+ Field for High-Field and Ultra-High-Field MRI

Abstract

A novel electromagnetic excitation method – the Huygens’ cylinder – is proposed to improve the B ₁ ⁺ field homogeneity of the high-field (HF) and ultra-high field (UHF) magnetic resonance imaging (MRI). Based on the concept of the Huygens’ box, we calculate the currents on a cylindrical boundary that can synthesize an arbitrary electromagnetic wave inside the enclosed region. Specifically, we excite a right-handed circularly polarized (B ₁ ⁺ ) travelling wave with high mode purity inside the Huygens’ cylinder coil. The simulated B ₁ ⁺ field obtained from several 3T and 7T MR scenarios are reported and compared with birdcage coils. In the unloaded scenarios, the Huygens’ cylinder achieves superior B ₁ ⁺ -field homogeneity over both the sagittal and axial plane compared to the birdcage coil for both 3T and 7T MRI. In the loaded scenarios, the Huygens’ cylinder achieves superior B ₁ ⁺ -field homogeneity over the sagittal plane and comparable B ₁ ⁺ -field homogeneity over the axial plane for both 3T and 7T MRI compared to the birdcage coil. Moreover, the 7T Huygens’ cylinder can generate a uniform field over a much larger region, enabling the imaging of a large part of the human body. The Huygens’ cylinder greatly improves the homogeneity of B ₁ ⁺ field and is free from the dielectric resonance limitation suffered by conventional RF coils. It has strong potential as future RF coils in HF and UHF MR systems.