Intravascular ultrasound (IVUS) constitutes a valuable technique for the diagnosis and management of coronary atherosclerosis. The detection of lumen and media-adventitia borders in IVUS images represents a necessary step in the utilization of the IVUS data for the reliable quantitative assessment of atherosclerotic lesions. In this paper, a fully automated technique for the detection of lumen and media-adventitia boundaries is developed. This comprises two different steps for contour initialization, one for each corresponding contour of interest, and a procedure for the refinement of the detected contours based on Radial Basis Function approximation. The proposed approach is shown to be capable of performing quick and reliable automated segmentation of IVUS images.