Double-Row 16-element Folded-End Dipole Transceiver Array for 3D RF Shimming of the Human Whole Brain at 9.4 T.

Homogeneity and longitudinal coverage of transmit (Tx) human head RF coils at ultra-high field (UHF, >7T) can be improved by 3D RF shimming, which requires using multi-row Tx-arrays. Examples of 3D RF shimming using double-row UHF loop transceiver (TxRx) and Tx-arrays have been described previously. Dipole antennas provide unique simplicity and robustness while offering comparable Tx-efficiency and SNR to conventional loop designs. Single-row Tx and TxRx human head UHF dipole arrays have been previously described by multiple groups. Recently, we developed a novel type of dipole antennas, a folded-end dipole, and presented single-row 8-element array prototypes for human head imaging at 7T and 9.4T. These studies have shown that the novel antenna design can improve the longitudinal coverage and minimize peak local SAR as compared to common unfolded dipoles. In this work, we developed, constructed, and evaluated the 16-element double-row TxRx folded-end dipole array for human head imaging at 9.4T. To minimize cross-talk between neighboring dipoles located in different rows, we used transformer decoupling, which decreased coupling to a level below -20 dB. The developed array design was demonstrated to be capable of 3D static RF shimming and can be potentially used for dynamic shimming using parallel transmission. For optimal phase shifts between the rows, the array provides 11% higher SAR-efficiency and 18% higher homogeneity than the folded-end dipole single-row array of the same length. The design also offers a substantially simpler and more robust alternative to the common double-row loop array with ~10% higher SAR-efficiency and better longitudinal coverage.