A catheter comprising a unitary guidewire and single mode optical fiber is used to illuminate human body channels, such as a tissue mass within a blood vessel, for purposes of diagnosis. A low coherent light source illuminates the tissue and provides a reference beam. Illumination that is reflected from the tissue is combined with the reference beam in an interferometer process. The path length of the reference beam is changed in a known manner in order to provide a known reference to the distance inside of the tissue from which reflected radiation is being received, thus providing information as to both the nature and the geometry of the tissue. A high signal to noise ratio is provided.