Unshielded Bent Folded-End Dipole 9.4 T Human Head Transceiver Array Decoupled Using Modified Passive Dipoles
Common decoupling methods, which require electrical connections, are difficult to use for decoupling of distantly located adjacent transmit (Tx) dipoles. Alternatively, neighboring dipoles can be decoupled using passive dipole antennas placed between them. This decoupling practice should be used with care since passive dipoles may interact destructively with the RF field, B1+, produced by the Tx array. In this work, we developed a novel decoupling method of adjacent Tx dipoles by using modified passive dipole antennas.
Coupling between two Tx dipole antennas produces two modes. A common parallel passive dipole placed between two Tx dipoles interacts only with the parallel mode. Therefore, such passive dipole resonating higher than both modes can shift down only the frequency of the parallel mode, which is adjusted by varying the passive dipole resonance frequency. Degeneracy of the two modes corresponds to decoupling of the Tx dipoles. To minimize destructive interference, we turned the passive dipole by 90º. Such modified passive dipole interacts only with the antiparallel mode. By varying the passive dipole resonance frequency, the frequency of the antiparallel mode can be shifted up till the Tx dipoles are decoupled.
The new perpendicular decoupling antennas produced substantially less destructive interference with the RF field of the array than the common parallel design. The constructed eight-element dipole array demonstrated good decoupling and whole-brain coverage.