Optical coherence tomography (OCT): Optical coherence tomography (OCT) uses a light source to image superficial tissues. The light reflected from superficial tissues (1-2 mm) is used to create an image similar to ultrasound image, but at much higher resolution as the wave length of light is much shorter than that of the ultrasound beam. The inner structure of the blood vessels of the heart can be studied in detail using OCT.