1. A computer implemented method for tracking a blood vessel centerline in a digital medical image, the method performed by the computer comprising the steps of: selecting a first blood vessel centerline point in an anatomical image, said anatomical image comprising a plurality of intensities associated with a 3-dimensional grid of points; providing a time series of one or more magnetic resonance (MR) flow images and MR magnitude images of said blood vessel, wherein each said flow image for a given time point comprises a plurality of 3-dimensional flow vectors associated with a 3-dimensional grid of points, and each said magnitude image for a given time point comprises a plurality of flow magnitudes associated with said 3-dimensional grid of points; for said centerline point, selecting an MR flow image for a time point wherein a flow magnitude at said centerline point in said MR flow image is a maximum over the time series of images; determining a direction of said centerline in said anatomical image from a direction of a flow vector associated with said centerline point in said flow image; and selecting, in said anatomical image, a new blood vessel centerline point displaced from said first vessel centerline point along said centerline direction.