الفهرس 
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Abstract
Advances in imaging of the cornea in vivo, have provided new methods for studying pathological and post-surgical changes in the optical and biomechanical properties of the cornea. Imaging of the cornea is critical to improve the diagnosis, to assess the severity and progression, and to evaluate the management of corneal diseases. Advances have been rapid, with improvement in hardware (such as light sources &imaging chips), optics (such as adaptive aberrations compensation) &soft ware (such as image tracking). Advances in optical imaging techniques include confocal microscopy, using Scheimpflug principle and optical coherence
tomography.
Confocal microscopy is a remarkable modification of reflected light microscopy. Principle of confocal microscope depends on passing light through an aperture to be focused by a condenser (illumination system) within a small area of the cornea. Reflected light then passes through an objective (observation system) that has the same focal point as the condenser. In doing so, light contamination from out of focus structures is eliminated; thus, bringing axial resolution to an order of 5-10 p.m and lateral resolution to 1-2 [im. In turn, this enables possible
magnification levels of approximately 600 times.
The confocal microscope allows non invasive high magnification imaging of the different layers of the cornea. Confocal microscopy could be used in imaging of different corneal diseases. Also it allows imaging of subbasal nerve plexus, which previously could not be observed with the slit-lamp biomicroscope or any other clinical technique.
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Summary
The main advantage of confocal microscopy is its high image magnification which allows corneal examination at the cellular level. Disadvantages of confocal microscopy include limited field-of-view. However, modern in vivo confocal microscopes overcame such limitation by moving the condenser to scan across the cornea. Also, confocal microscopes require corneal contact which may be uncomfortable to the patient and limits its early postoperative use.
Imaging technique based on Scheimpflug principle is used for the anterior segment of the eye. A camera perpendicular to a slit beam creates an optical section of the cornea and lens. More recent instrumentation has been designed to rotate around the visual axis capturing multiple images
to create a 3D image.
It has been used for the assessment of keratoconus, corneal clearance, corneal implants and corneal thickness. Scheimpflug measures of central corneal thickness (CCT) are accurate and have good repeatability compared with other contact pachymetry techniques.
Advantages of Scheimpflug based imaging technique include; being non contact simple, user-independent and quick technique. It provides a reliable, easy to use, method of measuring CCT. The disadvantage of scheimpflug imaging technique is its inability to differentiate the corneal layers one from the other.
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Summary
Optical coherence tomography (OCT) is a non contact imaging technology based on the principle of low-coherence interferometry. OCT allows in vivo cross-sectional imaging of tissue (tomography), a major advance over techniques such as photography and scanning laser ophthalmoscopy. Anterior segment OCT with 1310 nm illumination source is required for imaging the anterior segment structures due to penetration and tissue absorption light characteristics.
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. The limited resolution and clarity of image in time-domain OCT has been avoided in spectral domain OCT.
Anterior segment OCT has been used in the evaluation of corneal diseases and corneal refractive surgeries. Also, OCT is valuable in measurement of corneal thickness in refractive surgeries and in glaucoma suspects. Another application of anterior segment OCT is evaluation of corneal wound construction and healing after cataract surgery.