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

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.