Decoupling of Folded-End Dipole Antenna Elements of a 9.4 T Human Head Array Using an RF Shield
Due to the substantially smaller size of the sample, dipoles must be shorter than /2. In addition, head arrays are commonly placed on the surface of rigid helmets made sufficiently large to accommodate various heads. As a result, dipoles are not well loaded and are often poorly decoupled, which compromises the transmit (Tx) performance. Commonly, adjacent elements of a Tx-array are decoupled by circuits galvanically connected to both elements. Placement of such circuits between distantly located dipoles is difficult unless they are bent to bring their ends closer to each other. The latter restricts the choice of array geometries.
In this work, we developed a novel method of decoupling of adjacent folded-end dipole antennas using an RF shield located closely to the folded portion of the dipoles. We also used this technique while constructing a 9.4 T human head 8-element transceiver array. Since a pair of loaded dipole antennas is coupled capacitively, a mutual inductance has to be created to decouple dipole elements. The presence of the shield is equivalent to placing a mirror image of the dipole carrying an opposite current at a distance double of the distance to the shield. Folded portions of both dipoles (a real dipole and its mirror image) produce a capacitive bridge, which depends on the distance to the shield and the folded length. Thus, by adjusting the distance to the shield and folded length, we can compensate the intrinsic capacitive coupling. The constructed array demonstrates good decoupling and whole-brain coverage.