1. A method for characterizing vascular irregularity of a vessel, comprising the steps of: (a) determining an outline of said vessel in a cross sectional image, wherein said outline represents the inner wall of said vessel encompassing the vessel lumen or said outline represents the outer wall of said vessel encompassing mural thrombus; (b) determine an area of said outline; (c) defining a shape with substantially the same area as said outline; (d) fitting said shape to said outline; (e) calculating an irregularity index which is the ratio of at least a part of said outline and the outline of said fitted shape corresponding to said at least a part of said outline; and (f) visualizing said irregularity index.