The coax monopole antenna: A flexible end‐fed antenna for ultrahigh field transmit/receive arrays

• Published in: Magnetic resonance in medicine Vol. 92; no. 1; pp. 361 - 373
• Main Authors: Budé, Lyanne M. I., Steensma, Bart R., Zivkovic, Irena, Raaijmakers, Alexander J. E.
• Format: Journal Article
• Language: English
• Published: United States Wiley Subscription Services, Inc 01.07.2024
• Subjects: Antenna arrays; Coaxial cables; Computer Simulation; Coupling; Design optimization; Design parameters; Dipole antennas; Engineering; Equipment Design; Finite difference time domain method; Humans; Image Processing, Computer-Assisted - methods; Image quality; Inductors; Magnetic Resonance Imaging - instrumentation; Male; Matching; Monopole antennas; Phantoms, Imaging; Prostate; Prostate - diagnostic imaging; RF coil arrays; Simulation; ultrahigh field MRI; Engineering; RF coil arrays; ultrahigh field MRI
• ISSN: 0740-3194; 1522-2594; 1522-2594
• DOI: 10.1002/mrm.30036

Purpose The coax monopole antenna is presented for body imaging at 7 T. The antenna is fed at one end, eliminating the possibility of cable‐coil coupling and simplifying cable routing. Additionally, its flexibility improves loading to the subject. Methods Like the coax dipole antenna, an interruption in the shield of the coaxial cable allows the current to extend to the outside of the shield, generating a B1+ field. Matching is achieved using a single inductor at the distal side, and a cable trap enforces the desired antenna length. Finite difference time domain simulations are employed to optimize the design parameters. Phantom measurements are conducted to determine the antenna's B1+ efficiency and to find the S‐parameters in straight and bent positions. Eight‐channel simulations and measurements are performed for prostate imaging. Results The optimal configuration is a length of 360 mm with a gap position of 40 mm. Simulation data show higher B1+ levels for the coax monopole (20% in the prostate), albeit with a 5% lower specific absorbance rate efficiency, compared to the fractionated dipole antenna. The S11 of the coax monopole exhibits remarkable robustness to loading changes. In vivo prostate imaging demonstrates B1+ levels of 10–14 μT with an input power of 8 × 800 W, which is comparable to the fractionated dipole antenna. High‐quality images and acceptable coupling levels were achieved. Conclusion The coax monopole is a novel, flexible antenna for body imaging at 7 T. Its simple design incorporates a single inductor at the distal side to achieve matching, and one‐sided feeding greatly simplifies cable routing.