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An aneurysm of the abdominal aorta (AAA) is characterized by modified wall properties, and a balloon-like area usually filled by a thrombus. A rupture of an aortic aneurysm can be fatal, yet there is no way to accurately predict such an occurrence. The study of the wall and thrombus cross-sectional distension, due to a pressure wave, is important as a way of assessing the degradation of the mechanical properties of the vessel wall and the risk of a rupture. Echo ultrasound transverse cross-sectional imaging is used here to study the thrombus and the aortic wall distension, requiring their segmentation within the image. Polar coordinates are defined, and a search is performed for minimizing a cost function, which includes a description of the boundary (based on a limited series of sine and cosine functions) and information from the image intensity gradients along the radii. The method is based on filtering by a modified Canny-Deriche edge detector and then on minimization of an energy function based on five parts. Since echoes from blood in the lumen and the thrombus produce similar patterns and speckle noise, a modified version for identifying the lumen-thrombus border was developed. The method has been validated by various ways, including parameter sensitivity testing and comparison to the performance of an expert. It is robust enough to track the lumen and total arterial cross-sectional area changes during the cardiac cycle. In 34 patients where sequences of images were acquired, the border between the thrombus and the arterial wall was detected with errors less than 2%, while the lumen-thrombus border was detected with a mean error of 4%. Thus, a noninvasive measurement of the AAA cross-sectional area is presented, which has been validated and found to be accurate.