Investigation of a newly diagnosed occluding venous thrombus with duplexsonography, using contrast agent and compare the degree of vascularization in the same patient after 3 weeks and 3 months. The relative signal intensity difference (baseline to peak) of the time intensity curve (TIC) is measured in defined region of interests (ROI). Comparison of the visibility of revascularisation between color duplexsonography, power mode and contrast agent will be done.