Anterior segment OCT
Pablo Artal in Handbook of Visual Optics, 2017
Corneal topography can be revealed by mapping the corneal thickness (pachymetry), surface elevation, or curvature/power mapping (keratometry) and plays an important role in the assessment of keratorefractive surgical procedures, corneal transplantation, or screening the corneal degenerations and dystrophies (e.g., keratoconus). There are several techniques enabling topographic description of the eye (Mejia-Barbosa and Malacara-Hernandez, 2001, Pinero, 2015). Access to volumetric data sets enables mapping the corneal thickness (corneal pachymetry). Several studies have been performed to show reproducibility of OCT pachymetry and agreement with standard instruments (Wirbelauer et al., 2002, Li et al., 2006, 2008, 2010, Pinero et al., 2008). The comparison of corneal pachymetry for normal subject and patient with keratoconus is presented in Figure 4.18 (Karnowski et al., 2011). The maps were calculated from corresponding 3-D data sets after segmentation of corneal interfaces and correction for light refraction.
SKILL Corneal shape, structure and thickness
Sam Evans, Patrick Watts in Ophthalmic DOPS and OSATS, 2014
The shape and clarity of the cornea, the chief refracting structure of the eye, are paramount to ensuring the effective transmission of light to the neurosensory retina. The ability of the cornea to transmit light is affected by its microstructure, and its ability to focus light onto the retina is affected by its shape and surface quality. Modern imaging techniques are available to measure these properties and help the clinician make diagnoses and plan management. Basic examinations include corneal topography, corneal-thickness measurement and corneal biomicroscopy. The trainee should have a firm grasp of the techniques, their execution and interpretation.
The cornea
Mary E. Shaw, Agnes Lee in Ophthalmic Nursing, 2018
Corneal topography is a non-invasive medical imaging technique for mapping the surface curvature of the cornea. The anterior surface of the cornea, with its tear film, is the major refractive element in the eye and, because of this, even minute changes in its shape can reduce vision. Such vision-altering shape changes include irregular astigmatism induced by trauma and surgery, contact lens warpage and tear film irregularities.
The Association between Fourier Parameters and Clinical Parameters in Myopic Children Undergoing Orthokeratology
Published in Current Eye Research, 2021
Di Wang, Dejia Wen, Bin Zhang, Weiping Lin, Guihua Liu, Bei Du, Fang Lin, Xiaorong Li, Ruihua Wei
Corneal topography is commonly used to evaluate and monitor the corneal shape and refractive status. The corneal topography’s circular nature makes it a perfect candidate for Fourier analysis, which decomposes a complex pattern into the combination of a series of sine waves of different frequencies. The direct current component represents the mean values of the refractive power. The sine wave that runs one cycle per 360 degrees describes the corneal asymmetry. The sine wave that runs two cycles over 360 degrees describes regular astigmatism. Any other pattern would indicate irregular corneal astigmatism.18 Two of the most often used topography system, Pentacam and Tomey, have equipment Fourier analysis capacity. The method has been applied to the assessment of the optical quality of cornea in normal eyes,19–21 eyes with an ocular condition such as keratoconus and pterygium,22,23 and eyes after corneal surgeries,24–26 lens surgeries,27,28 and retinal detachment surgery.29 After orthokeratology, over one-third of eyes had corneal asymmetry beyond the normal range.30 Therefore, it is easy to see why Fourier analysis in orthokeratology.
Comparison of Javal-Schiøtz keratometer, Orbscan IIz and Pentacam topographers in evaluating anterior corneal topography
Published in Clinical and Experimental Optometry, 2023
Jennifer M Turner, Christine Purslow, Paul J Murphy
Ocular surface topography (OST) is a commonly used technique for an accurate and reliable measurement of the ACS.11 Elevation-based corneal topography relies on the measuring instrument being able to produce an accurate representation of the ocular surface profile.2 This depends on the availability of a precise mathematical function for the best interpretation of the observed corneal images. Topographic maps are then rendered to provide a visual picture of the small surface curvature variations seen when comparing normal and abnormal surface shapes. These curvature maps subtract the surface profile from a best-fit sphere, providing the height difference between the two in a colour-coded representation. This allows the viewer to identify areas of discrepancy and observe any patterns of progressive change.
Recent developments in keratoconus diagnosis
Published in Expert Review of Ophthalmology, 2018
Marcella Salomão, Ana Luisa Hoffling-Lima, Bernardo Lopes, Michael W. Belin, Nelson Sena, Daniel G. Dawson, Renato Ambrósio
Placido-disk-based corneal topography characterizes the anterior or front surface of the cornea using quantitative data to generate color-coded maps [37]. Different topographical indexes, such as Rabinowitz and McDonnell’s, were proposed for detecting keratoconus [43]. Such data, while often lacking specificity, have proved to be sensitive to recognize mild ectatic patterns, even in patients with relatively normal distance spectacle-corrected visual acuity and unremarkable slit lamp exams [44,45]. This ability has historically positioned corneal topography as a mandatory exam in the screening process of refractive surgery candidates [16]. However, the limitations of limiting the analysis to the anterior curvature are realized when considering cases that developed post-refractive keratectasia, despite normal anterior curvature [46-48]. There are also patients with abnormal topographic maps preoperatively, but proceeded with LCV based on advanced corneal imaging and have documented stable outcomes [20]. Interestingly, the subjective evaluation of corneal topography maps has important limitations, including high variability among experts and also within the same expert with different color scales [49]. These situations demonstrate the need for the rational use of objective data in order to enhance sensitivity and specificity when considering ectasia diagnosis and screening patients for LVC.
Related Knowledge Centers
- Cataract Surgery
- Cornea
- Eye
- Visual Perception
- NON-Invasive Procedure
- Medical Imaging
- Ophthalmology
- Optometry
- Intraocular Lens
- Refractive Surgery