  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.
