Five ml of the contrast agent sulfur hexafluoride (= SonoVueTM, Bracco International B.V., Amsterdam, the Netherlands) is given intravenously into a superficial vein of the dorsal foot or calf of the leg investigated (green butterfly needle 20 gauge). If no leg veins are available the arm vein route will be used. The measurements are done in the fully occluding thrombus area in cross section. The area will be localized in order to find the same point of investigation again at visit 2 and visit 3.