Magnetoinductive metasurface of capacitively-loaded split rings for local field homogenization in a 7 T MRI birdcage:...

The transmit field B₁⁺ in a 7 T birdcage is inherently inhomogeneous due to the effects of wavelengths on tissue. This work investigates the homogenization of this field through metasurfaces that consist of a two-dimensional planar array of capacitively loaded conducting rings. The metasurfaces are placed in the intermediate space between the head and the birdcage on either side of the head. The periodical structure of this type of metasurface supports magnetoinductive waves because of the mutual inductive coupling existing between the elements of the array. The analysis takes advantage of this coupling and exploits the excitation of a standing magnetoinductive wave across the arrays, which creates a strong local field that contributes to locally homogenize the field of the birdcage. The presence of the arrays does not detune the birdcage, so that they can be used with commercial birdcages that operate both to transmit and to receive.