Instrument Tracking and Navigation for MRI-guided Interventions

Abstract

Interventional MRI requires accurate and fast localization of medical instruments within the imaging volume of the MR scanner. Furthermore, in view of tissue motion and target dislocation, accurate intra-operative imaging is demanded. The research presented in this thesis addresses these issues with reference to a proposed MRI-guided transrectal prostate biopsy system. As the instrument is not visible in the MR images, RF fiducial markers embedded within the instrument are used to determine its pose. A novel localization method to compute the location of N fiducial markers using 1D projections is presented. The method is shown to yield significant improvements over previously proposed methods. Computational complexity was significantly reduced by avoiding cluster analysis, while high accuracy was achieved by using a set of optimally chosen projections and by applying Gaussian interpolation in peak detection. The method was analyzed and validated using a combination of experiments and Monte Carlo simulations. Experiments in 1.5 T and 2.9 T MR scanners involved both water phantoms and volunteer subjects. High robustness and sub-pixel accuracy were demonstrated while the computational time showed an improvement of up to a factor of 100 over existing solutions. This method was employed as the basis for tracking the endorectal probe during the prostate biopsy procedure. The probe was positioned by means of a remotely actuated manipulator. Miniature semiactive markers were embedded within the probe in a rigid known geometrical configuration and tracked by means of the localization method. At each position, Least-Squares fitting of the probe model with the localized one was performed in order to achieve more accurate tracking. Navigation of the probe and biopsy needle was realized through a dedicated graphical user interface. This interface displayed interpolated cross sections through the MR imaging volume and simplified graphical models of the instruments overlaid on the anatomy. Visual guidance was further improved by filtering of the markers' positions, which was enabled by the high tracking rate. In order to improve intra-operative imaging a novel external receiver array was designed and a prototype was built, as an alternative to the more conventional endorectal and pelvic receivers. This new array coil was optimized for imaging of the prostatic area for a patient in the prone position by combining a buttery coil and three single trapezoidal loops. The design is suitable for positioning the endorectal probe and does not introduce any spatial limitation to the range of movements. Experiments in a 1.5 T MR scanner and simulations demonstrated higher receiver sensitivity and homogeneity than conventional coils and also a significantly improved signal-to-noise ratio.