Flexible and shape-adjustable coaxial capacitor (COCA) coils for ultrahigh field MRI: a comparative analysis with rigid coils

Flexible RF coils enhance patient comfort and increase filling factors, making them attractive for MRI. However, achieving first-mode resonance at 7 T for large-diameter, flexible coils remains a challenge. We present a coaxial capacitor (COCA) coil, which can be 10 cm in diameter and still operates in the first resonant mode at 298 MHz. Unlike coaxial cable coils that rely on self-resonance, the COCA coil combines ultrasoft Litz wire for inductance with a short coaxial structure for capacitance. Bench tests showed that a 1-capacitor COCA coil provides comparable tuning/matching robustness, effective detuning, and inter-element decoupling performance to conventional rigid coils with three distributed lumped capacitors. MRI acquisitions demonstrated high SNR, especially when the coil conformed to the curvature of the load, with up to 20 % SNR improvement over flat configurations. The coil's ability to retain tuning and matching across different shapes also supports the development of shape-adjustable arrays. By enabling flexible, large-diameter coils to operate in the first resonant mode at ultrahigh fields, the COCA design offers a promising solution for imaging anatomies with complex geometries, such as the shoulder, foot, and spine.