RF coil element with longitudinal and transversal two-peak field distribution for low SAR 7-Tesla magnetic resonance imaging

In ultra high field MRI (B₀ ≥ 7 T) the allowed local SAR restricts the power that can be applied to the RF coil. Here in this paper we propose an approach where the power that is absorbed by the body is better distributed, yielding a lower SAR_max value, hence higher power level can be used for feeding. The optimized SAR distribution is excited by an extended dipole (41 cm-long), which is longer than the ordinary half-wavelength one, and an eigen-resoanant metal plate used for shielding purpose. At 300 MHz, the excited dipole has an approximate 3λ/2 electrical length where the maximum of current is located away from the feeding position, exciting a longitudinal two-peak SAR distribution, whereas the passive shielding plate exhibits a half-wavelength eigen-resonance, which yields a transveral two-peak SAR distribution. Compared to an established strip line element (25 cm-long), a 29% improvement of figure of merit (H_mean/√SAR_max) is obtained based on simulation.