THE OPTICAL COHERENCE TOMOGRAPHY

“Quick, Safe and Informative”, three words can simply used to describe on one of the imaging instrument that widely use in eye professional care. The Optical Coherence Tomography or OCT in short, is new retina scanner. The OCT is a new diagnostic tool that performs tomography or cross sectional imaging of biologic tissues with less and equal to ten microns axial resolution using light waves. Furthermore, OCT can be classified as the first imaging technique that provides information regarding the retinal tomography that is akin to in vivo histopathology of the retina. The existences of OCT give best information about retinal imaging that is complementary to the conventional topographic techniques or fundus. Meaning that, OCT yields tomography imaging that provides information of each layers of the retina. There are two types of OCT which are the Ultrahigh Resolution OCT and the Standard Resolution OCT. The Ultrahigh Resolution OCT has the ability to identify fine retinal structures such as the external limiting membranes and ganglion cell layer, whereas the Standard Resolution OCT is not visualized as clearly in compared to ultrahigh resolution OCT.


Function of the Optical Coherence Tomography
The function of the Optical Coherence Tomography is mainly to diagnose any macular disease especially exist in retina layer. Among disease that can be diagnosed by using the OCT are holes, cystoids oedema, epiretinal membranes, vitreomacular traction, central serous retinopathy and etc. The other vital functions are: to monitor progression of disease and response to treatment of patient, to have great picture to differentiate between long standing retinal detachment and retinochisis. The retinochisis is a vitreoretinal degeneration characterized by splitting of the retina into two layer; either splitting of the nerve fiber layer from retina or splitting at the outer plexiform layer, while the retinal detachment is separation of retina from the pigment epithelium layer. To analysis the optic nerve head and retinal nerve fiber layer thickness. Besides, the OCT also can yield the imaging of the anterior segment; the space within the eye filled with aqueous humour and bounded anteriorly by the cornea and posteriorly by the iris. Besides, the OCT procedure is able to reduce or eliminate the need for fluorescein angiography for some patients. The fluorescein angiography is a technique aimed at observing the vessels of the fundus of the eye and iris by using photography following the intravenous injection of flourescein.


Principle of the Optical Coherence Tomography
The principle of the Optical Coherence Tomography is generally similar with the principle on which ultrasound works where the high frequency sound wave is launched into the eye with the help of a probe. But, the working principle of OCT has two major differences. The first, the OCT uses light as the probe rather than ultrasound wave. The main reason is the speed of light is almost a million times faster than sound. Thus, it allows the measurement of structures with resolution of lower and equal to ten microns compared to hundred microns scale for ultrasound. The second, ultrasound need contact with the tissues interest, whereas OCT does not requires any contact.


Optics of the Optical Coherence Tomography
The optics of Optical Coherence Tomography consists of light sources, interferometer, reference mirror and photo detector. The OCT works begin with a broad width near infrared light beam (820nm) is projected on to the retina. The light gets reflected from the boundaries between the microstructures and also get scattered differently from tissues or layer with different optical properties. It then compares the echo time delay of the light that is reflected from the various layers of the retina with the echo time delay of the same wavelength that is reflected from reference mirror at known distance. The interferometer then combines the reflected pulses from the retina as well as reflecting mirrors and resulting in a phenomenon known as interference. This interference is then measured by a photo detector, which determines the distance travelled by various echoes by varying distance to the reference mirror. This finally produces a range of time delay for comparison. The interferometer integrates several data points over 2mm of depth to construct a tomogram of the retinal structures. The first tomogram imaging use false color scale and later different color inserted to represent degree of light backscattering from different depths of retina. The basic color appears in retina tomogram are classified in three categories. The first appear as bright color of red, yellow and white; represent high reflectivity structures. The examples of layer that appear with bright colors are nerve fiber layer and plexiform layer. The second category is darker colors that represent low reflectivity structures with blue and black color appearance. The examples of layer that appear with darker color are inner nuclear layer and outer nuclear layer. The third category is intermediate reflectivity structures that appear with green color and the examples of this category are inner plexiform layer and outer plexiform layer. The image that produced has axial resolution of below and equal to ten microns and transverse resolution of twenty microns.