High-field magnetic resonance imaging (MRI) ($\ge 3\mathrm{T}$) is a powerful imaging modality that enables high-resolution visualization of biological tissues. However, at such field strengths, MRI is limited by spatial inhomogeneities in the transmit radio-frequency (rf) magnetic field caused by wave interference. Here, we demonstrate a planar resonant structure composed of concentric split-ring resonators (SRRs) for tunable control of the rf magnetic near field at 123 MHz. By introducing frequency offsets between individual resonators, the relative amplitudes of induced currents are redistributed, reshaping the local magnetic field profile and amplitude within the structure. Numerical simulations and experiments demonstrate tunable magnetic field distributions with improved uniformity. The proposed approach provides a compact and passive route toward reconfigurable rf field control for improved high-field MRI performance.

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• Medical magnetic resonance imaging
• Near-field optics
• Optoelectronics
• Split-ring resonators
• Tissues
• Magnetic resonance imaging
• Radio frequency techniques