optical coherence tomography

IntroductionOCT (optical coherence tomography) is a novel noninvasive, noncontact imaging modality which produces high depth resolution (10 microns) cross-sectional tomographs of tissue. It is similar to ultrasound, except that optical rather than acoustic reflectivity is measured. Its value is given by the possibility of achieving pseudo-histological images of the target tissue. It is optically based, analogue to ultrasound B-scan examination and similar to laser reflectometry. Optical coherence tomography involves shining low-level infrared light on a tissue specimen, an interferometer and a computerized imaging system. Because of its high resolution, non-ionizing radiation, etc., in recent years some scholars use it in medical field. This essay will state theory, applications, Advantages and limitations of OCT.System principleLasers transmitting in the chaos medium will be scattering and absorption, and the intensity, coherence and polarization of the light will be changed. According to the number of incidence photons’ scattering, photons can be divided into 3 types: ballistic photon, snake-like photon and scattering photon. Ballistic photons, without scattering through the medium, retain coherence and lots of internal information of scattering medium. Scattering photons have multiple scattering; only bring a small amount of scattering media information. The initial characteristics of the photon are lost, especially coherence. Snake-like photons only have a small number of scattering, transmit with a small incident angle at the direction of axis, retain most of the characteristics of incident photons and have part of structure information of the media. OCT uses the medium information which carried by ballistic photon and the snake-like photon to make image.The core of the OCT system is a fiber-optic Michelson interferometer. Coupling light is transmitted into single-mode fiber. Output is divided by 3dB coupler into two. One of them through the confocal lens system and focused on the sample. The other one through the lens expand and irradiate to the reference mirror. It is used to be reference light. Two beams of light couple and re-join. As Optical path difference less than the coherent light length, the samples of the ballistic light and snake-like light happen to interfere. Interference signal DC circuit transmits through photomultiplier tube and amplifier, collected and processed by a computer. The two-dimensional data we get directly stand for record the organization of the scattering of incident light in samples of the situation regarding the depth and lateral position of function. The results of the data array directly treat as a gray-scale.ApplicationsOptical coherence tomography (OCT) has developed rapidly since its potential for applications in clinical medicine was first demonstrated in 1991. OCT performs high-resolution, cross-sectional tomographic imaging of the internal microstructure in materials and biologic systems by measuring back scattered or back reflected light. Mathematical models have been used to promote understanding of the OCT imaging process and thereby enable development of better imaging instrumentation and data processing algorithms. One of the most important issues in the modeling of OCT systems is the role of the multiple scattered photons, an issue, which only recently has become fully understood; the works of Thrane et al. and Turchin et al. representing the most comprehensive modeling. This technology was used in ophthalmic examination of the retina before. Later, was used for skin and gastrointestinal tract, such as study of incompletely transparent tissues. Until now, we can see the reports of the OCT for the field of stomatology. Colston reported for the first time, in 1998, used OCT technology to get the OCT image of the pig’s premolar dentin and periodontal tissues, Baumgartner in the same year has successful gotten clear OCT images of cementum sector, Amaechi, in 2001, has gotten a typical OCT images of caries tissues. However, these studies are limited to isolated teeth study. Specifically in the future, the success of the OCT detector for nonnasality study will make the OCT images of teeth-in-mouth possible.Results and interpretationsOptical coherence tomography is a new mode of high-resolution tomography, with optical technology and ultra-sensitive detector combined together, application of modern computer image processing, developed into an emerging diagnostic techniques tomography. Since 2001, at the first report of OCT technology in human coronary artery to obtain high-resolution images, the reports on OCT technology in the field of application of percutaneous coronary intervention are gradually increased. Now, the experts all over the world are pay attention to it. OCT has the following advantages: (1) it combines advantages of confocal and weak coherence, optical tomography scan, and has high detection sensitivity and resolution; (2) non-ionizing radiation, safety environmental protection; (3) No damage can be detected; (4) to adopt the optical communications fiber-optic components, it is cheap and portable. However, the shortcomings of the present are: (1) it should not distinguish between the lesion tissues with similar density; (2) incident factors can affect the imaging results; (3) image acquisition time is too long, short term getting a large number of images is still difficult; (4) image analysis software to be further researched and developed, and the lack of different races, different age groups of the normal database.ConclusionBased on the aspects above, it can be concluded that with the rapid development of biomedical Photonics technology, OCT technology will have greater scope for development, technology improvements will focus on the improvement of resolution and imaging depth on the increase. OCT is a young technology. Though it still has little limitations, we can not deny, OCT will become a common and effective means of medical diagnosis.ReferencesFroehly, L & Furfaro, L 2009, ‘Dispersion compensation properties of grating-based temporal-correlation Optical Coherence Tomography systems’, OPTICS COMMUNICATIONS, vol. 282, no. 7, pp1488-1495.Hillman, E &Burgess, S 2009, ‘Sub-millimeter resolution 3D optical imaging of living tissue using laminar optical tomography’, LASER & PHOTONICS REVIEWS, vol. 3, no. 1-2, pp 159-179.Rosenthal, N & Guagliumi, G 2009, ‘Comparison of Intravascular Ultrasound and Optical Coherence Tomography for the Evaluation of Stent Segment Malapposition’, JOURNAL OF THE AMERICAN COLLEGE OF CARDIOLOGY, vol. 53, no. 10, pp A22-A22, NEW YORK.Wolfgang, D & James, G 2008, ‘modeling Light–Tissue Interaction in Optical Coherence Tomography Systems’, Biological and Medical Physics, Biomedical, pp73-115.。