Comparison of MEMS switches and PIN diodes for switched dual tuned RF coils

Purpose: To evaluate the performance of micro-electromechanical systems (MEMS) switches against PIN diodes for switching a dual-tuned RF coil between F-19 and H-1 resonant frequencies for multi-nuclear lung imaging. Methods: A four-element fixed-phase and amplitude transmit-receive RF coil was constructed to provide homogeneous excitation across the lungs, and to serve as a test system for various switching methods. The MR imaging and RF performance of the coil when switched between the F-19 and H-1 frequencies using MEMS switches, PIN diodes and hardwired configurations were compared. Results: The performance of the coil with MEMS or PIN diode switching was comparable in terms of RF measurements, transmit efficiency and image SNR on both F-19 and H-1 nuclei. When the coil was not switched to the resonance frequency of the respective nucleus being imaged, reductions in the transmit efficiency were observed of 32% at the F-19 frequency and 12% at the H-1 frequency. The coil provides transmit field homogeneity of +/- 12.9% at the H-1 frequency and +/- 14.4% at the F-19 frequency in phantoms representing the thorax with the air space of the lungs filled with perfluoropropane gas. Conclusion: MEMS and PIN diodes were found to provide comparable performance in on-state configuration, while MEMS were more robust in off-state high-powered operation (>1 kW), providing higher isolation and requiring a lower DC switching voltage than is needed for reverse biasing of PIN diodes. In addition, clear benefits of switching between the F-19 and H-1 resonances were demonstrated, despite the proximity of their Larmor frequencies.