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 ¹⁹ F and ¹ H 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 ¹⁹ F and ¹ H 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 ¹⁹ F and ¹ H 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 ¹⁹ F frequency and 12% at the ¹ H frequency. The coil provides transmit field homogeneity of ±12.9% at the ¹ H frequency and ±14.4% at the ¹⁹ F 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 ¹⁹ F and ¹ H resonances were demonstrated, despite the proximity of their Larmor frequencies.