This paper introduces a technique for the automated description of tubular objects in 3 0 medical images. The goal of automated 30 object description is to extract a representation which consistently details the location, size, and structure of objects in 3 0 images using minimal user interaction. Such a representation provides a means by which objects can be classijied, quantijiably evaluated, and registered.It also serves as a region of interest specification for visualization processes.The technique presented in this paper is suited f o r generating representations of 30 objects with nearly circular cross sections which have, possibly as a result of a global operation (e.g., blurring), intensity extrema near their centers. Such tubular objects commonly occur within human anatomy (e.g., vessels and selected bones). The medial axis of each of these objects is well approximated by its intensity ridge. The scales of the local maxima in medialness at all points along the ridge can be mapped to local width estimates. Together these measures capture the location, size, and structure of tubular objects. This paper covers the mathematical basis, the implementation issues, and the application of this technique to the extraction of vessels from 30 magnetic resonance angiographic images and bones from 3 0 X-ray computed tomographic images.