Bent Folded-End Dipole Head Array for Ultra-High-Field Magnetic Resonance Imaging Turns “Dielectric Resonance” from an Enemy to a Friend.

A) Simulated transversal map of the Hz (parallel to the cylinder axis) and Et (tangential) components of the RF field, generated inside the elliptical phantom by the 8-element bent folded dipole array with and without the RF shield. Frequency dependence (“resonance curve”) of the RF field of the phantom TE mode simulated for different sizes of the RF shield (B).

Clinical application of ultra-high field (UHF, >7T) MRI, is still hindered due to technical hurdles associated with imaging of large human objects (body, head). Resulting issues include a significant increase of the local tissue heating (peak SAR (pSAR)), and a strong inhomogeneity of the transmit (Tx) RF magnetic field, B1+. Improvement of the B1+ homogeneity and coverage can be achieved by using multi-loop Tx-arrays. However, an adequate whole-brain coverage was demonstrated only by using multi-row arrays capable of 3D RF shimming.

Comparison of B1+ maps and images (GRE, MP2RAGE) obtained by the 8-element folded-end dipole (A) and 16-loop (B) arrays for the same male subject.

In this work, we developed a novel human head 9.4 T transceiver array consisting of eight bent folded-end dipole antennas. We demonstrate that in the presence of the RF shield, the developed array simultaneously excites two modes, i.e. a circular polarized mode of the array itself, and the TE mode (“dielectric resonance”) of the human head. Mode mixing can be easily controlled by changing the length of the folded portion. Due to this mixing, the new dipole array improves longitudinal coverage as compared to unfolded dipoles. By optimizing the length of the folded portion, we also minimized the pSAR value by ~50%. The new array also provided better whole-brain coverage compared to o a more complex 16-element double-row surface loop array. Overall, the novel design approach improved the Tx-performance and simplified the head coil design making it more robust and, therefore, safer.

EM simulated models (A) of different 8-element dipole arrays and corresponding central sagittal SAR10g (B) and B1+ (C) maps.

Avdievich NI, Solomakha G, Ruhm L, Bause J, Scheffler K, Henning A. Bent:
Folded-end Dipole Head Array for Ultra-High-Field Magnetic Resonance Imaging Turns “Dielectric Resonance” from an Enemy to a Friend.
Magn Reson Med2020;84:3453–3467.
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