Decoupling of folded‐end dipole antenna elements of a 9.4 T human head array using an RF shield

• Published in: NMR in biomedicine 2020-09, Vol.33 (9), p.e4351-n/a
• Main Authors: Avdievich, Nikolai I., Solomakha, Georgiy, Ruhm, Loreen, Scheffler, Klaus, Henning, Anke
• Format: Article
• Language: English
• Subjects: Antenna arrays; array optimization, decoupling of dipole antennas, human head transceiver array, transmit performance, ultra‐high field MRI, whole‐brain coverage; Arrays; Biological products; Circuits; Decoupling; Dipole antennas; Head; Magnetic resonance imaging; Protective equipment; Radio frequency shielding
• ISSN: 0952-3480; 1099-1492
• DOI: 10.1002/nbm.4351

Dipole antennas have recently been introduced to the field of MRI and successfully used, mostly as elements of ultra‐high field (UHF, ≥ 7 T) human body arrays. Usage of dipole antennas for UHF human head transmit (Tx) arrays is still under development. Due to the substantially smaller size of the sample, dipoles must be made significantly shorter than in the body array. Additionally, head Tx arrays are commonly placed on the surface of rigid helmets made sufficiently large to accommodate tight‐fit receive arrays. As a result, dipoles are not well loaded and are often poorly decoupled, which compromises Tx efficiency. Commonly, adjacent array elements are decoupled by circuits electrically connected to them. Placement of such circuits between distantly located dipoles is difficult. Alternatively, decoupling is provided by placing passive antennas between adjacent dipole elements. This method only works when these additional components are sufficiently small (compared with the size of active dipoles). Otherwise, RF fields produced by passive elements interfere destructively with the RF field of the array itself, and previously reported designs have used passive dipoles of about the size of array dipoles. In this work, we developed a novel method of decoupling for adjacent dipole antennas, and used this technique while constructing a 9.4 T human head eight‐element transceiver array. Decoupling is provided without any additional circuits by simply folding the dipoles and using an RF shield located close to the folded portion of the dipoles. The array reported in this work demonstrates good decoupling and whole‐brain coverage. A novel method of decoupling dipole antenna elements of a human head transmit array at 9.4 T is developed. Decoupling is realized by folding the dipoles and placing an RF shield near the folded portion, which produces additional mutual inductive coupling between them. The method allows decoupling of distantly located dipole antennas. An eight‐element single‐row transceiver array was constructed and evaluated. The array provides full‐brain coverage and good decoupling, substantially better than obtained by unfolded dipole arrays.