A Novel Intravascular MRI Coil with optimized Sensitivity

Objective: In the past, various configurations have been proposed for intravascular imaging coils. Single loop coils [1] are simple to construct, but suffer from a very inhomogeneous spatial sensitivity. Opposed solenoid coils [2] and meander coils [3] solve this problem at the cost of a quick drop-off of the sensitivity in radial direction. A variant of Helmholtz coils (Fig.1) has been proposed and optimized for active tracking of catheters [4]. Based on the Helmholtz design, a novel miniaturized coil has been developed for intravascular imaging and tracking. The main objective of this work is to optimize the spatial sensitivity of the coil for intravascular imaging. Simulation Methods: It was the objective of the simulations to obtain a coil geometry that provides a radially symmetric spatial sensitivity in the central plane of the coil. Under the assumption that the coil length is sufficiently large compared with the thickness of the imaging slice at the central plane, the Biot-Savart-field of the coil in that plane was calculated analytically in 2D using MATLAB. Infinitesimal small current elements on the surface of a cylinder (∅1.67 mm, 5 F) were used to represent the coil windings. This 2D simulation approach was chosen to keep the simulation effort manageable. The number of the windings and their azimuthal position was varied. The azimuthal variation of the coil sensitivity was minimized at a radial distance of 0.25 mm from the catheter surface. The field distribution becomes more homogeneous at larger distances. Finally, an FEM simulation (COMSOL) was used to verify the field calculations in MATLAB and to evaluate the effect of the finite size of the windings (150 x 50 μm) for the optimum geometry found in MATLAB.