An electromagnetic MEMS-based resonator design for catheter tracking in MRI

Abstract

Using Magnetic Resonance Imaging (MRI) in minimally invasive medical procedures has many advantages, such as the elimination of ionizing radiation and increased soft tissue contrast. However, the lack of MRI-compatible catheters is a signi cant issue for MRI-based endovascular applications. In this thesis, we present design, fabrication, and characterization of MEMS resonator structures, which are intended to be used in a catheter tracking system for MRI. The actuation mechanism is based on Lorentz force, where a driving current carries the position information for the catheter. Novelty of this work is the utilization of the huge DC magnetic eld present in MRI for actuation, which leads to a low-power solution. Moreover, using optical techniques to transfer the signal to the outside of the MRI environment eliminates the use of metal wires, which cause heating problem in MRI. We designed devices for two di erent optical readout methods, namely laser Doppler vibrometry (LDV) and di raction grating interferometry (DGI). The necessary theoretical background for the design along with the analytical calculations is given in this thesis. Dynamic characteristics of resonators are also investigated by FEA simulations. A three-mask procedure is developed to fabricate the devices and the details of the fabrication process in cleanroom are explained. We achieved to de ne grating elements with 4 m period and 50 % ll factor, which is the highest resolution required for the fabrication of the devices. In characterization stage, the dynamic characteristics of the fabricated devices are obtained. The results show that the devices can indeed be actuated by Lorentz force and the produced mechanical signal can be detected using optical methods. We measured the signal-to-noise ratio to be at least 20 throughout the frequency range of interest, for 0.13 T magnetic eld strength and 0.4 mA rms driving current.