The segmentation of objects from complex images is difficult due to indistinct boundaries between objects and similarity of objects. We have used a hierarchical segmentation approach to accurately distinguish between objects and identify the corresponding boundaries. This approach has been used successfully to extract the aorta from transverse magnetic resonance (MR) images of the abdomen. The procedure to segment the abdominal aorta involves three progressive steps: aorta detection, aorta extraction, and estimation of the aorta wall boundary. Comparison of hierarchical segmentation techniques with single-step segmentation methods (e.g., region-growing, edge-detection) shows that hierarchical segmentation yields more reliable results.