Dr Nikolai I. Avdievich | Improvement of Central SNR and Transmit Coverage of a Human Head Phased Array at Ultra-High Field Using Dipole Antennas

The first part of the presentation deals with an improvement of the central SNR of human head array at ultra-high magnetic fields (UHF, > 7T). Increasing the number of surface loops in a human head receive (Rx) array improves the peripheral signal-to-noise ratio (SNR), while SNR near the brain center doesn’t substantially change. Recent theoretical works demonstrated that an optimal central SNR at UHF requires contribution of two current patterns associated with a combination of surface loops and dipole antennas. Use of various dipole antennas as MRI RF detectors has been recently introduced and successfully implemented mostly for imaging human body sized objects. In this work, we evaluated and compared several Rx dipole-like elements for use within human head UHF Rx-array. We constructed and characterized novel single-row and double-row phased arrays, which consisted of transceiver (TxRx) surface loops and Rx-dipoles. We demonstrated that combining surface loops and dipole-like elements substantially (> 30%) improve SNR near the brain center as compare to arrays consisted of surface loops only. The second part of the presentation discusses an improvement of the transmit (Tx) coverage of the human head array coils. Due to a substantial shortening of the RF wave length (below 15 cm at 7 T), RF magnetic field at UHF has a specific Tx excitation pattern with strongly decreased (more than 2 times) values at the periphery of a human head. This effect is seen not only in the transversal slice but also in the coronal and sagittal slices, which considerably limits the longitudinal Tx-coverage (along the magnet’s axis) of conventional surface loop head arrays. In this work, we developed a novel human head UHF array consisted of 8 TxRx folded dipole antennas circumscribing a head. Due to an asymmetrical shape of dipole elements, the array couples to the intrinsic “dielectric resonance” mode of the head. Due to this interaction, firstly, the new array provides for a simple way of minimizing the maximum local SAR. Secondly, it provides for a longitudinal coverage better than that achieved by a similar array consisted of unfolded dipoles as well as by an 8-element single-row and 16-element double-row surface loop arrays.