Bent folded-end dipole head array for ultrahigh-field MRI turns "dielectric resonance" from an enemy to a friend

Purpose To provide transmit whole-brain coverage at 9.4 T using an array with only eight elements and improve the specific absorption rate (SAR) performance, a novel dipole array was developed, constructed, and tested. Methods The array consists of eight optimized bent folded-end dipole antennas circumscribing a head. Due to the asymmetrical shape of the dipoles (bending and folding) and the presence of an RF shield near the folded portion, the array simultaneously excites two modes: a circular polarized mode of the array itself, and the TE mode ("dielectric resonance") of the human head. Mode mixing can be controlled by changing the length of the folded portion. Due to this mixing, the new dipole array improves longitudinal coverage as compared with unfolded dipoles. By optimizing the length of the folded portion, we can also minimize the peak local SAR (pSAR) value and decouple adjacent dipole elements. Results The new array improves the SEE (< B-1(+)>/root pSAR) value by about 50%, as compared with the unfolded bent dipole array. It also provides better whole-brain coverage compared with common single-row eight-element dipole arrays, or even to a more complex double-row 16-element surface loop array. Conclusion In general, we demonstrate that rather than compensating for the constructive interference effect using additional hardware, we can use the "dielectric resonance" to improve coverage, transmit field homogeneity, and SAR efficiency. Overall, this design approach not only improves the transmit performance in terms of the coverage and SAR, but substantially simplifies the common surface loop array design, making it more robust, and therefore safer.