A 32-element loop/dipole hybrid array
To improve whole-brain SNR at 7 T, a novel 32-element hybrid human head array coil was developed, constructed and tested. Our general design strategy was based on two major ideas. Firstly, following suggestions of previous theoretical works, we combined loops and dipoles for improvement of SNR near the head center. Secondly, we minimized the total number of array elements by using a hybrid combination of transceive (TxRx) and receive (Rx) elements. The developed hybrid array consisted of eight folded-end TxRx-dipole antennas and three rows of twenty-four Rx-loops all placed in a single layer on the surface of a tight-fit helmet.
The new array significantly improved SNR in vivo both near the center (~20%) and at the periphery (20 to 80%) in comparison to a common commercial array coil with 8 transmit (Tx) and 32 Rx-elements. While twenty-four loops alone delivered central SNR very similar to that of the commercial coil, the addition of complementary dipole structures provided further improvement. The new array also provided ~15% higher Tx-efficiency and better longitudinal coverage than that of the commercial array.
In conclusion, the developed array coil demonstrated advantages in combining complementary TxRx and Rx resonant structures, i.e. TxRx-dipoles and Rx-loops all placed in a single layer at the same distance to the head. This strategy improved both SNR and Tx-performance, as well as simplified the total head coil design making it more robust.