High-Gain Feed Antenna for Improved Travelling-Wave Excitation Efficiency in Magnetic Resonance Imaging

Magnetic Resonance Imaging (MRI) is currently one of the most promising medical imaging techniques. At ultra-high static magnetic field strength (≥7 T), MRI achieves improved signal-to-noise ratio and enhanced image quality to resolve fine anatomical structures at submillimeter scale. Nevertheless, at these high field strengths, the wavelength of the MRI radiofrequency (RF) field becomes smaller than the human body dimensions. As a result, conventional birdcage transmits resonators exhibit inherent standing wave effects, leading to RF field inhomogeneity and higher specific absorption rate (SAR). Previous research has demonstrated that employing travelling-wave excitation through an embedded waveguide can effectively address the issues of RF field inhomogeneity and high SAR associated with traditional birdcage coils in ultra-high field MRI. However, travelling-wave excitation suffers from low energy transmission efficiency when exciting the waveguide, and there is a lack of clear design principles for feed antenna. This paper explores the correlation between the antenna gain and proton spin excitation efficiency through exciting waveguide in ultra-high field travelling-wave MRI. Novel designs for the feed antenna are proposed as well. It suggests additional space of improvement in travelling-wave excitation at ultra-high field MRI.