Transmit Power Optimization for Tracking, Wireless Marker and Imaging applications of a Multi-mode Endovascular coil

Introduction: In previous work, we introduced the concept of the multi-mode intravascular MRI coil [1,2] which consists of a single active device that is connected to the external system via a single coaxial cable and performs three functions: 1) active tip tracking, 2) imaging, and 3) inductively-coupled wireless marker. The purpose of such a multi-functional coil is to enable single insertion; wholly MRI-guided minimally invasive procedures. As shown in figure 1, the multi-mode coil consists of a tightlywound tracking solenoid with a highly localized sensitivity pattern that is connected in series with an elongated imaging loop that has a broader sensitivity pattern. Furthermore, the multi-mode coil is inductively coupled to the external transmit coil in order to enable the wireless marker functionality as described in [3]. This is achieved by transforming the open circuit at the decoupling circuitry to a low impedance at the coil terminals via a (2n-1)λ/4 transmission line, thereby allowing current induced by the transmit field to flow in the multi-mode coil. This results in the magnification of the transmit B1 field within the region of sensitivity of the multi-mode coil, leading to larger flip angles compared to regions outside the coil sensitivity. In addition, the magnification of the B1 field due to the tracking component of the multi-mode coil is greater than that due to the imaging component due to the greater conductor (and hence, current) density of the tracking solenoid. The dominant tracking peak may be used to localize the position of the catheter tip using 3 orthogonal projections. The objective of this work is to determine the optimal transmit power for each mode of operation of the multi-mode endovascular coil.