Recognition and tracking of magnetic nanobots using MRI

By switching the gradient fields of a clinical magnetic resonance imaging (MRI) scanner, magnetic objects may be moved inside the cardiovascular system of the human body. The main field of application is seen in targeted drug therapy or embolization. A successful navigation of such devices requires continuous position determination. The occurrence of magnetic susceptibility artifacts can be exploited for this purpose. This article studies the effect of magnetic microscopic objects and nanoparticles on the process of MRI image formation in several imaging sequences. An MRI simulator based on evaluation of the Bloch equation is presented and applied for the simulation of artifact formation. Also, artifact properties are studied by experiments carried out on clinical MRI scanners, using magnetic objects placed into an agarose gel phantom. The transferability of the results from the gel phantom to a real tissue environment is proven. Based on the results, a two-stage procedure for visual servoing is proposed. It is initialized by object detection, carried out in a 3D scan. Object tracking is performed on fast 2D scans by template matching. The slice position is adjusted automatically in a feedback loop in order to follow object movements perpendicular to the image plane.