ABSTRACT
OCT is very similar to ultrasound imaging, but the difference is that OCT imaging uses light rather than sound. In ultrasound imaging, high-frequency ultrasound is directed toward the sample with an ultrasonic probe transducer, and the sound wave is reflected back from the inner structures having various acoustic properties. The ultrasonic probe detects the reflected sound, and the measurements of internal structures are estimated from the echo delay. In OCT, a beam of light is reflected onto the sample material and the backscattered or back-reflected light from the internal structure is measured to determine the microstructure and properties of the sample materials. The major difference between OCT and ultrasound is that the velocity of light (3 × 108 m/s) is a million times faster than sound (1500 m/s in water), and hence measurement of distance using light needs ultrafast time resolution. Direct electronic detection is impossible in this range of time, hence OCT measurements of echo time delay are dependent on correlation techniques. Ultrasound imaging needs direct contact with the material to be imaged or immersion of the material in liquid or other medium for transmission of sound waves, whereas OCT could be performed without making any contact to the material or without any requirement for a transducing medium (Fujimoto and Brezinski, 2003).
