Hilbert fractal inspired dipoles for passive RF shimming in ultra-high field MRI

Ultra-high field MRI head coils present a characteristic B₁⁺ magnetic field distribution resulting in inhomogeneous signal and contrast over the image, affecting relevant regions of interest such as the temporal lobes of the brain and the cerebellum. This is a consequence of the spatially varying flip angle distribution attributed to the reduction of the electromagnetic wavelength inside the human tissues. Without radical changes in the experimental setup, this problem has been effectively targeted by different passive RF shimming approaches such as high permittivity dielectric pads or metamaterials. The latter, however, may potentially decrease the B₁⁺ field in other relevant areas or compromise the patient's comfort. Here, we present a novel approach based on meander dipoles inspired from Hilbert fractals. The structures were designed and studied numerically using finite element simulations. Prototypes of the structures were printed and tested with a 1Tx/32Rx birdcage head coil on a 7 T MR scanner. We demonstrate a new device based on compact, thin and flexible design, able to improve the B₁⁺ field over each temporal lobe without deterioration of the RF performances in other brain areas and with minimal impact on patient comfort.