An apparatus for Doppler interferometric imaging comprising a balanced fiber optic interferometer, a low coherence light source and an imaging guidewire is used for diagnosis of intravascular obstructions. The imaging guidewire, which is also used for placement of over-the-wire catheters, comprises a hollow elongated body, an optical fiber, a rotatable mirror at its distal end and a micromotor operable to sweep the sampling beam around the vessel. An optical length modulator in the reference arm of the interferometer provides ramp wise modulation of optical length and a Doppler frequency shift for the reference beam. The polarization diversity detection scheme and two envelope detectors tuned for the Doppler frequency, perform measurements of the intensities of the two linear polarizations of the retroreflected light. Combined with current values of the optical length of the reference arm and angular position of the sweeping mirror, this information is used for reconstruction of the cross-sectional images of the vessel in two polarizations. These images are used for measurements of the geometry and recognition of the morphology of an intravascular obstruction which is utilized for diagnosis of atherosclerosis and real-time guidance of interventional procedures like PTCA.