The invention describes a novel implementation of ultrasound or OCT techno-logy to approximate the dimensions of fluid-filled structures (when using ultrasound techno-logy) or other structures (when using OCT technology). The invention in a preferred embodi-ment is an elongated member such as a catheter that uses ultrasound or OCT technology to ap-proximate the dimensions of a structure into which the catheter has been placed. In a preferred embodiment, the catheter includes multiple ultrasound transducers arranged in an annular or circumferential configuration on, embedded into or within the body of the elongated member so that distance measurements can be obtained between the elongated member and the wall of the immediately facing structure (e.g., fluid-filled lumen): Utilizing these measurements, the present invention approximates for the physician the shape and size of the structure into which the elongated member is placed. The invention also includes a method for producing three-di-mensional images from two-dimensional Images.