Optical Coherence Tomography Technology
Since Huang et al. from the Massachusetts institute of technology at the United States published an article titled “technology of optical coherence Tomography (OCT)” on Science in 1991, OCT technology has been an active field of scientific research. OCT can obtain high-resolution cross-sectional imaging of tissue microstructure. Similar to ultrasonic imaging, OCT uses infrared light waves to replace sound waves to focus beam into the tissue to measure at different axial and lateral position repeatedly, which obtain image information to get the two-dimensional backward scattering field or reflection images. OCT image reflects the structure of the tissue and cellular structure [1, 25, 30, 48, 63].
OCT technology combines the confocal, low coherence, optical heterodyne and scanning tomography and other technical advantages, which can realize real-time, non-invasive and in vivo detection. It has a high detection sensitivity and resolution: lateral resolution can reach 4-2 m, the longitudinal resolution can reach 10 p,m, far greater than the resolution of the X-ray photography. In the clinical imaging, OCT integrated with catheter or endoscopic can get high resolution imaging of the internal organs microstructure. Applications in medical include: imaging of articular cartilage, cardiovascular imaging, imaging of the esophagus, cervix imaging, retinal imaging, etc. It also can be used to measure the characteristic parameters of biological tissue and fluid velocity.
Based on low-coherence interferometry, Optical coherence tomography is typically employing near-infrared light, which allows penetration into tissue to collect scattering light from layers with different depths. OCT is coherent field tomography technology, its interference conditions are: (1) the frequency of the two beams is the same or very close to, namely light frequency difference is much smaller than the frequency of light used; (2) phase difference of two beams of light is a constant; (3) light polarization direction is not perpendicular to each other.
The center part of the OCT is a Michelson interferometer, as shown in Fig.2.10. A coherent light source is fed into a 2 x 2 optical fiber coupler, which connects to reference side (a mirror) and the signal side (sample to be tested), respectively. Mirror reflected light (the reference light) interferes with light backscattered from the sample (signal light) through the fiber coupler to produce interference signal, which is received by a detector. The intensity of the signal reflects the scattering (reflection)
Fig. 2.10 System schematic of OCT strength of the sample, only the scattering signal from a particular sample depth is coherent with the reference beam due to a short coherent length of the light utilized, thus a high layer selectivity, or depth resolution is obtained. Tomographic resolution is directly determined by the coherence length of the light source, the shorter the coherence length, the higher the depth resolution. But the shorter the coherence length of the light source, the weaker its interference signal. In choosing a light source, one should consider a balance among the resolution, the optical properties of the sample, and appropriate light source coherence. To achieve the lateral resolution, output light beam needs to be focused down to micrometer size for 2D scanning. The performance of OCT imaging capability also depends on sensitive heterodyne detections and discrimination of scattered light from off-focal planes. By use of a PS-OCT system with an integrated retinal tracker, analysis of optimum conditions for depolarization imaging, data processing, and segmentation of depolarizing tissue in the human retina was realized .