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.

A) Modes of a pair of coupled dipoles. B) General representation of a decoupling effect produced by a common parallel (top) and modified perpendicular (bottom) passive dipoles (red). EM simulation models of 8-element bent folded-end dipole arrays without decoupling (C) as well decoupled using three methods, i.e. the common parallel passive dipoles (D) (decoupling 1), common parallel passive dipoles moved away from the sample (E) (decoupling 2), and modified perpendicular passive dipoles (F) (decoupling 3).

A) Modes of a pair of coupled dipoles. B) General representation of a decoupling effect produced by a common parallel (top) and modified perpendicular (bottom) passive dipoles (red). EM simulation models of 8-element bent folded-end dipole arrays without decoupling (C) as well decoupled using three methods, i.e. the common parallel passive dipoles (D) (decoupling 1), common parallel passive dipoles moved away from the sample (E) (decoupling 2), and modified perpendicular passive dipoles (F) (decoupling 3).

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.

A) Central transversal B1+ maps obtained using decoupled bent folded-end arrays. B) Ratios of corresponding B1+ maps to that obtained without decoupling. C) Central sagittal maps of the ratios of B1+ fields obtained using decoupled bent (unfolded) arrays to that obtained by the array without decoupling. — A) Central transversal B₁⁺ maps obtained using decoupled bent folded-end arrays. B) Ratios of corresponding B₁⁺ maps to that obtained without decoupling. C) Central sagittal maps of the ratios of B₁⁺ fields obtained using decoupled bent (unfolded) arrays to that obtained by the array without decoupling.

A) Central transversal B₁⁺ maps obtained using decoupled bent folded-end arrays. B) Ratios of corresponding B₁⁺ maps to that obtained without decoupling. C) Central sagittal maps of the ratios of B₁⁺ fields obtained using decoupled bent (unfolded) arrays to that obtained by the array without decoupling.

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.

Avdievich NI, Solomakha G, Ruhm L, Henning A., Scheffler K.:

Unshielded Bent Folded-End Dipole 9.4 T Human Head Transceiver Array Decoupled Using Modified Passive Dipoles.

Magn Reson Med 2021;86:581–597.