An apparatus and associated method for detecting vulnerable plaque within a lumen defined by an intraluminal wall is described. The apparatus includes a probe having a distal portion and a proximal portion. The apparatus includes an optical waveguide extending along the probe. The optical waveguide is configured to carry optical radiation between the distal and proximal portions, and has a distal end in communication with the intraluminal wall. The apparatus includes an interferometer coupled to the optical waveguide and configured to provide an interference signal for sub-surface imaging of the intraluminal wall, and a processing module configured to provide spectroscopic information from detected intensity of light collected from the intraluminal wall.