Corneal topography
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[[Image:Corneal topography right ax.jpg|right|thumb|A corneal topogram. Blue shows the flattest areas, and red the steepest]]
'''Corneal topography''', also known as '''photokeratoscopy''' or '''videokeratography''', is a [[Non-invasive (medical)|non-invasive]] [[medical imaging]] technique for mapping the surface curvature of the [[cornea]], the outer structure of the [[eye]]. Since the cornea is normally responsible for some 70% of the eye's [[refractive power]],<ref name="isbn0-7817-6512-9">
{{cite book
|author=Pavan-Langston, Deborah
|title=Manual of Ocular Diagnosis and Therapy
|publisher=Lippincott Williams & Wilkins
|location=Hagerstwon, MD
|year=
|pages=p405
|isbn=0-7817-6512-9
|oclc=
|doi=
}}</ref> its topology is of critical importance in determining the quality of [[Visual perception|vision]]. The three-dimensional map is therefore a valuable aid to the examining [[ophthalmologist]] or optometrist and can assist in the diagnosis and treatment of a number of conditions; in planning [[refractive surgery]] such as [[LASIK]] and evaluation of its results; or in assessing the fit of [[contact lens]]es. A development of [[keratoscopy]], corneal topography extends the measurement range from the four points a few millimeters apart that is offered by [[keratometry]] to a grid of thousands of points covering the entire cornea. The procedure is carried out in seconds and is completely painless.
=== Method of operation ===
The patient is sat facing a bowl containing an illuminated pattern, most commonly a series of concentric rings. The pattern is focused on the anterior surface of the patient's cornea and reflected back to a [[digital camera]] at the centre of the bowl. The topology of the cornea is revealed by the shape taken by the reflected pattern. A computer provides the necessary analysis, typically determining the position and height of several thousand points across the cornea. The [[topographical map]] can be represented in a number of graphical formats, such as a ''sagittal map'', which color-codes the steepness of curvature according to its [[dioptre|dioptric value]].
=== Development ===
The corneal topograph owes its heritage to 1880, when the Portuguese ophthalmologist [[Antonio Placido]] viewed a painted disk (''Placido's disk'') of alternating black and white rings reflected in the cornea.<ref name=goss>{{cite journal | author= Goss D, Gerstman D| title= The Optical Science Underlying the Quantification of Corneal Contour: | journal= Indiana Journal of Optometry| year= 2000| volume= 3| issue= 1| pages= | url= http://www.opt.indiana.edu/IndJOpt/ijosum00.pdf}}</ref> The rings showed as [[contour line]]s projected on the corneal [[epithelium]]. In 1896, [[Allvar Gullstrand]] incorporated the disk in his [[ophthalmoscope]], examining photographs of the cornea via a microscope and was able to manually calculate the curvature by means of a numerical [[algorithm]]. Gullstrand recognized the potential of the technique and commented that despite its laboriousness it could "give a resultant accuracy that previously could not be obtained in any other way".<ref>{{cite journal | author= Gullstrand A | title= Procedure of rays in the eye | journal= Helmholtz’s Treatise on Physiological Optics| year= 1909| volume= | issue= | pages= | url= }}</ref> The flat field of Placido's disk reduced the accuracy close to the corneal periphery and in the 1950s the Wesley-Jessen company made use of a curved bowl to reduce the field defects.<ref name=goss/> The curvature of the cornea could be determined from comparison of photographs of the rings against standardized images.
In the 1980's, photographs of the projected images became hand-digitized and then analysed by computer. Automation of the process soon followed with the image captured by a digital camera and passed directly to a computer.<ref>Busin M, Wilmanns I, Spitznas M. ''Automated corneal topography: computerized analysis of photokeratoscope images.'' Graefes Arch Clin Exp Ophthalmol. 1989;227(3):230-6. PMID 2737484</ref> In the 1990s, systems became commercially available from a number of suppliers. The first completely automatic system was the Corneal Modeling System (CMS-1) developed by Computed Anatomy, Inc. in New York City. The price of the early instruments was initially very high ($75,000), largely confining their use to research establishments. However, prices have fallen substantially over time, bringing corneal topographs into the budget of smaller [[clinic]]s and increasing the number of patients that can be examined.
== References ==
* {{cite book | first = | last =Yanoff M, Duker J | authorlink = | coauthors = | year = 2004 | month = | title = Ophthalmology| chapter = | editor = | others = | edition =2nd Edition | pages = | publisher = Mosby | location = | id = ISBN 0-323-01634-0| url = }}
=== Further reading ===
* {{cite book | first = | last =Corbett M, O'Brart D, Rosen E and Stevenson R| authorlink = | coauthors = | year = 1999| month = | title = Corneal Topography| chapter = | editor = | others = | edition = | pages = 230| publisher = BMJ Books| location = London| id = ISBN 0-7279-1226-7 | url = }}
== Notes ==
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[[Category:Ophthalmology]]