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

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.

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.

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.

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.