Novel optically tunable capacitor for RF safe tuning and decoupling of intravascular MR coils

S. Weiss, T. Uelzen, G. Pantel, J. Mueller Philips Research Laboratories, Hamburg, Germany, Department of Micro Systems Technology, Hamburg University of Technology, Hamburg, Germany, MRI Devices Daum GmbH, Schwerin, Germany Objective: The motivation for the application of intravascular imaging coils is to gain high SNR images from a local area. To maximize SNR, proper tuning and matching of such coils is of utmost importance [1]. Moreover, active decoupling during transmit is important for image quality and RF safety of intravascular coils. In current interventional devices, tuning and matching is usually accomplished with fixed elements, which is a limitation, because the load and the shape of interventional coils may change as with expandable coils. Photodiodes have been used for optical decoupling of such coils [2,3], but this concept requires additional capacitors in series and parallel to the photodiode, which requires additional connections and complicates the tip assembly. Furthermore, the photodiode concept is not appropriate for tuning. Remote tuning and matching with varactors [4] or PIN diode based decoupling is problematic in interventional devices, because the required control leads are prone to RF heating. Recently, a transformer-based transmission line for the MR signal has been proposed that can vastly reduce RF heating of active interventional devices [5]. However, this transmission line does not transmit DC as required for varactor based tuning and PIN diode based decoupling. Here, we propose an optically tunable capacitor based on a dedicated metal-oxide-semiconductor interface for these purposes.