Aiming at problems of both the contour initialization and the robustness in traditional segmentation methods for Intravascular Ultrasound(IVUS) images,a novel scheme is proposed to segment IVUS images of plaques based on the active contour model and the Contourlet multiresolution analysis.With the contourlet transform,the original image is decomposed into lowpass components and bandpass directional subbands.The template matching is adopted in lowpass components to yield the initial contours of lumen border and media-adventitia border.The anisotropic diffusion is then utilized in bandpass directional subbands to suppress the noise as well as preserve vascular boundaries,and the contour evolution in the boundary vector field is used to obtain final contours.The method is evaluated via 100 simulated images and 120 real images,and results demonstrate that the mean distance error and the percentage of area differences have increased by 3.04 pixels and 6.30%,respectively,as compared with those of the traditional active contour model.These results show that the proposed method can automatically and accurately extract two vascular borders.