Abstract
The problem of detecting and tracking SPAMM tags has been the focus of cardiac MRI in the past few years. Among the many methods proposed to do that, the harmonic phase method (called HARP) has a large potential to enable real-time tracking. We propose two improvements to the original HARP technique to address the problem of magnetic field inhomogeneity and improve tracking. We derive a theoretical model for the problem of field inhomogeneity and propose an extension of the simulated phase evolution rewinding technique (SPHERE) whereby consecutive images are acquired using alternating echo time. Then, we propose a simple and computationally efficient approach that allows the problems of disappearing tag lines to be addressed. In particular, the geodesic horizontal and vertical distances between neighboring corner points in the detected tag lines are computed for each image. Once the correspondence between tag lines is established, the tracking of any point on the wall can be obtained given its relationship to the four corner points surrounding its location. The proposed improvements were implemented and applied to numerical phantoms as well as experimental data obtained for the original HARP technique to demonstrate the potential of the new techniques.