Page 42 - The UK Guide to Laser Eye Surgery
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42 Questions about technology
 Topography and Tomography
How does it feel?
You feel nothing as the instrument does not make contact with your eye.
How does it benefit you?
Topography and tomography are some
of the most important safety factors
in determining your suitability for
laser eye surgery. It is important that
both topography and tomography be performed at your preoperative screening. Topography (such as the Atlas or Keratron) provides the most accurate measurement of the front surface of your eye, while tomography (such as the Orbscan, Pentacam or Galilei) measures the shape of both the front and back surfaces of the cornea, and the thickness profile of your cornea. The information provided by this 3D map of your cornea is essential for determining whether there are any corneal abnormalities or if it is too thin for surgery to be safe. In patients with very irregular corneas, topography-guided treatments can be performed to achieve a more regular corneal shape.
What is it for?
Pachymetry is for measuring corneal thickness - specifically, a pachymeter measures the depth of the thinnest point of your cornea.
Hand-held pachymetry is traditionally used primarily to measure the central area of the cornea where the cornea is thinnest.
It takes single point measurements of the corneal thickness.
Corneal pachymetry can also be
mapped using optical devices such as the tomography scanners described above (Pentacam, etc). The gold standard for pachymetry measurement by
optical devices is an optical coherence tomography (OCT) scanner.
Very high-frequency (VHF) ultrasound
is the most precise way to measure the thickness of the cornea. In addition, it provides 3D thickness profiles of the individual layers of the cornea, allowing improved diagnostic capabilities for both pre-operative and post-operative analysis.
Both tomography and VHF ultrasound provide important information about the shape of the cornea that is used to screen for abnormalities such as keratoconus.

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