A Self-Matched Leaky-Wave Antenna for Ultrahigh-Field MRI with Low SAR

Abstract

The technology of magnetic resonance imaging is developing towards higher magnetic fields to improve resolution and contrast. However, whole-body imaging at 7 T or even higher fields remains challenging due to wave interference, tissue inhomogneities and high RF power deposition. Nowadays, proper RF excitation of a human body in prostate and cardiac MRI is only possible to achieve by using phased arrays of antennas attached to the body (so-called surface coils). Due to safety concerns, the design of such coils aims to minimize the local specific absorption rate (SAR) keeping the highest possible RF signal in the region of interest. All previously demonstrated approaches were based on resonant structures such as e. g.dipoles, capacitively-loaded loops, TEM-line sections. In this study, we show that there is a better compromise between the transmit signal and the local SAR using non-resonant surface coils due to weaker RF near fields in the close proximity of their conductors. With this aim, we propose and experimentally demonstrate a first leaky-wave surface coil implemented as a periodically-slotted microstrip transmission line. Due to its non-resonant radiation, the proposed coil induces only slightly over half the peak local SAR compared to a state-of-the-art dipole coil, but has the same transmit efficiency in prostate imaging at 7 T. Unlike other coils, the leaky-wave coil intrinsically matches its input impedance to the averaged wave impedance of body tissues in a broad frequency range, which makes it very attractive for future clinical applications of 7 T MRI.