An evaluation of the coax monopole antenna as a transmit array element for head imaging at 14 T

Purpose: In comparison to dipole antennas, the coax monopole antenna (CMA) diminishes the possibility of cable-coil coupling. This greatly facilitates cable routing in spatially restricted environments, such as head coil arrays. With the outlook of a 14T MRI system being installed at the Donders Center in Nijmegen, the Netherlands, this study aims to optimize the CMA for an eight-channel head array at 14 T and compare its performance with an array of fractionated dipole antennas. Methods: Both antenna designs were optimized for head imaging at 14 T using single-channel finite-difference time-domain (FDTD) simulations at 596 MHz. Eight-channel simulations were then used on a human model to evaluate (Formula presented.) and specific absorption rate (SAR) distributions. For both antenna types, prototype arrays were built by placing eight elements on a 26-cm-diameter cylindrical holder. These prototype arrays were used for S₁₁ and S₁₂ evaluation. Results: The optimal dimensions of the CMA were a length of 20 cm and a gap position of 4 cm. The fractionated dipole was optimal for a length of 25 cm. Evaluation of 100 000 random shims revealed that the CMA performs with lower SAR efficiency, although the SAR efficiencies are similar in CP mode. Measured S₁₁ and S₁₂ levels were both lower for the CMA. Conclusion: The coax monopole would be an excellent candidate for head coil arrays at 14T MRI. Although the CMA is expected to perform with lower SAR efficiency than the fractionated dipole, its single-ended design will facilitate elements placement and cable-routing, especially in a spatially restricted environment.