A modified optimization-based contour detection method was presented to compute the lumen area of the coronary artery from intra-vascular ultrasound (IVUS) video images. Firstly, the search range for the artery inner wall was determined based on the continuity of IVUS video frames. Then the internal and external energy were calculated to describe the smoothness of the artery wall and the grayscale variation of ultrasound images respectively. Here a novel form of the external energy, which combined the gradient and variance of the intensity of image in the radial direction, was used. Finally, the minimal energy path, which was the optimum contour of the artery wall, was obtained using a circular dynamic programming (DP) method. By the comparison with the typical DP procedure using the traditional external energy form, which only counted the image gradient, the reliability of this modified method is considerably improved in the measurement of the lumen area of the coronary artery.