الفهرس 
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Abstract
Optical coherence tomography (OCT) is a non contact imaging technology which provides detailed cross-sectional images (tomography) of biological tissues. It is based on the principle of low-coherence interferometry. The process is similar to that of ultrasonography, except that a low coherence near-infrared light emitted from a superluminescent diode is used instead of sound waves. Interferometry allows for measurement of microstructures as time delay of light signals reflected back from examined structures is compared to time delay of light reflected back from a reference mirror at specific distances.
One and one-half decades ago, ophthalmic OCT was developed for retinal imaging, using a near infrared 0.8-μm wavelength. Recently, OCT technology has been adapted for anterior segment imaging using an infrared light with longer wavelength (1.3 μm). This wave length has two advantages: First, because 1.3 μm light is absorbed by water, retinal exposure is much lower, allowing the use of twenty fold higher power without exceeding the retinal exposure limit. The higher power, in turn, permits much higher scan rates without sacrificing image quality. Second, the longer wavelength light scatters less in opaque tissues, allowing for deeper penetration. This permits imaging through the limbus to visualize structures of angle of anterior chamber.
Anterior segment (AS)-OCT imaging systems started with Slit-Lamp adapted equipments as experimental prototypes. In the last few years specially designed equipments were manufactured such as Visante AS-OCT.
Applications of AS-OCT include assessment of precorneal tear film, contact lens fitting and corneal opacities. Also, corneal surgical procedures can be evaluated with AS-OCT. Pre LASIK, corneal power & thickness can be measured accurately. While post LASIK, precise visualization and thickness measurement of both flap and residual stroma in addition to detection of LASIK complications can be achieved. Also, OCT is valuable in postoperative evaluation of PRK, PTK, keratoplasty, intrastromal corneal segments, and clear corneal cataract incisions.
Assessment of configuration of angle of anterior chamber and rapid screening for occludable angles can be achieved with high-resolution imaging of Anterior segment OCT. Also pupillary block and plateau iris syndrome can be differentiated. However, a current disadvantage is that AS-OCT can’t image the ciliary body behind the iris. In addition to its diagnostic role in glaucoma, AS-OCT is a promising tool for imaging postoperative morphology of trabeculectomy blebs. The obtained informations enhance evaluation of bleb function and may aid in making decisions regarding postoperative bleb management.
OCT imaging of crystalline lens can help in grading of cataract and in evaluation of dynamic accommodation. While in phakic IOL implantation, OCT has an important preoperative role as it can measure both depth and width of anterior chamber in addition to determination of crystalline lens rise. Also, high resolution imaging of OCT can detect postoperative complications of phakic and pseudophakic IOLs such as IOL calcification, IOL-iris contact, capsular block syndrome, capsule bending and posterior capsular opacification.
Regarding imaging of uveal tissues, AS-OCT can determine the exact site, size and content of some iris pathologies such as iris cysts and tumours. Also images of ciliary body and sclera may be obtained in such conditions.
AS-OCT coupled with beam splitter of operating microscope is used for non contact intraoperative visualization of anterior segment procedures. This technique allows high resolution, cross sectional imaging and pachymetry of the cornea and sclera. It is particularly helpful for lamellar dissection techniques such as deep lamellar keratoplasty and subscleral trabeculectomy.
In comparison with UBM, anterior segment OCT has the advantage of being a non contact technique in which no immersion saline is required. Also, OCT images have higher resolution and measurement of anterior segment dimensions is more accurate. However AS-OCT, can not image posterior chamber of the eye as done with UBM.