Robust real-time MR-geometric distortion correction for interventional procedure on mobile targets

Rapid dynamic MR-imaging can provide both functional and positional informations in real-time, which can be used to retroactively control an interventional procedure. Although rapid echo-planar imaging (EPI) is frequently used in this role, EPI often suffers from geometric distortions. These signal localization errors are caused by spatial magnetic field inhomogeneities induced by magnetic suceptibility variations within the human body and instrumental limitations of the acquisition system. This type of artifact can be addressed by mapping and converting the spatial magnetic field inhomogeneities a priori into a pixel shift and to subsequently correct the EPI-images. Furthermore, for real time guidance of interventions in organs subject to periodical physiological motion, the correction needs to account for time variant changes of the magnetic field during the motion cycle. The correction described in this paper consists of two steps: A parameterized model of the motion induced magnetic field perturbation is computed in a preparative step; subsequently, during the intervention, this model is used to reconstruct the magnetic field perturbation corresponding to the actual organ position in real-time, which in turns allows computation of distortion corrected MR-images.