Corneal Topography and 3D Software
The advent of computerised corneal topography analysis (through videokeratoscopes) has changed our level of understanding of the shape of the corneal surface.
Collection of corneal topography data has been made easier, but its interpretation
has become more complex. We are no longer confined to measuring the axial radius
of curvature of a small central region of the cornea as we do with a conventional
keratometer. 
We have been working for several years on different methods of both analysing and presenting corneal topography data.
With the software we have developed, we are able to import data from several currently available videokeratoscopes and then analyse and present the data in several ways. Scaling and size of the area for analysis can be adjusted, surface maps can be averaged and standard deviations of the averages can be calculated.
Just as it is difficult to get a good idea of the topography of a land surface with a two-dimensional map, so it is with the corneal topography map. It is much easier to visualise the corneal surface by showing it in a 3D (perspective) map. The 3D map can graphically show even small changes in shapes of the topography and give a better ‘solid object view’ of what is happening.
The following corneal surface shows a moderately severe case of keratoconus. The standard ‘top down’ view is presented here side by side with the 3D view. The corneal shape changes show up dramatically in the 3D format. There is a large inferior peak region (cone) below the visual axis, and the irregular astigmatism of the cornea is visible as the two peaks straddle the visual axis.
Read More:
- Buehren T, Collins MJ, Loughridge J, Carney LG, Iskander DR. Corneal topography and accommodation. Cornea 2003; 22(4): 311-316.
- Buehren T, Lee BJ, Collins MJ, Iskander DR. Ocular microfluctuations and videokeratoscopy. Cornea 2002; 21(4): 346-351.
- Buehren T, Collins MJ, Iskander DR, Davis B, Lingelbach B. The stability of corneal topography in the post-blink interval. Cornea 2001;. 20(8): 826-833.
High Speed Videokeratoscopy
Videokeratoscopes are clinical instruments for measuring the surface topography of the eye. We have modified a videokeratoscope to allow continuous recording at conventional video frame rates of 50 Hz (a significant improvement in time resolution compared with other published methods). The data we acquire is digitally stored and later analysed to produce a continuous record of ocular surface topography.
An example of high speed videokeratoscopy for a subject pulling their eyelid, as in the removal of an RGP lens, may be downloaded below. The video shows the dramatic effect that the force of the eyelids can have on the topography of the cornea. At 5 seconds into the recording, the subject pulls their eyelids outward (a similar action to that of removing a rigid contact lens from the eye). The corneal topography map shows a marked increase in with-the-rule astigmatism (i.e. the cornea becomes steeper along the vertical meridian and flatter along the horizontal meridian).
High speed videokeratoscopy: subject pulling their eyelid
Download the high speed videokeratoscopy video (8.37MB, Zip file)
The videokeratoscope is said to measure the topography of the cornea, however the reflected image that the videokeratoscope uses in the measurement is in fact reflected from the tear film and therefore provides the opportunity to study subtle changes in tear topography over time. This allows detailed analysis of tear film dynamics, including the tear film changes immediately before and after natural blinks. Blinking is a crucial factor in the distribution and replenishment of the tear film and by monitoring the tear dynamics during the blink cycle (approximately 5 -10 seconds elapses between each blink) we gain critical information about the interaction between tear dynamics and blinking. Understanding such dynamics is important to objectively determine the characteristics of tear stability, tear flow, tear film break-up, and in assessing clinical problems such as dry eye and contact lens surface dryness. The figure below shows the changes in tear film topography immediately following a blink. Notice that the tear topography becomes thicker at the top of the map and thinner at the bottom of the map. We assume that this reflects an upward flow of tears following the blink.
Ocular surface height difference maps for subject DZ, derived at t=0.02s, 0.22s, 0.42s... postblink. The tear film thickens at the top and thins at the bottom of the ocular surface. Time resolution = 0.2s. |
Read More:
- D. R. Iskander and M. J. Collins, “Applications of high speed videokeratoscopy”, Clinical and Experimental Optometry, Vol. 88, No. 4, 2005 (in press) [Email for copy: d.iskander@qut.edu.au]
- D. R. Iskander, M. J. Collins, and B. Davis, “Evaluating tear film stability in the human eye with high speed videokeratoscopy”, IEEE Transactions on Biomedical Engineering, 52(12), December 2005 (in press) [Email for copy: d.iskander@qut.edu.au]
Pupillometry
Determination of two-dimensional characteristics of the anterior surface of the eye is becoming increasingly important in modern optometry and ophthalmology practice. The pupil parameters change under different lighting conditions so they often need to be related to some fixed reference such as the limbus outline. However, current commercial pupillometers do not estimate limbus position. We have developed a novel algorithm for automatic extraction of pupil parameters from digital images that takes the relative limbus information into account. We apply the developed algorithm to images obtained by a standard digital camera, and specialized ophthalmic instruments such as a wavefront sensor and a high-speed imaging system.
High-speed pupillometry (subject with nystagmus)
A video demonstration of high-speed pupillometry may be downloaded here (4.05MB, Zip file).
Read more:
- D. R. Iskander, M. J. Collins, S. Mioschek, and M. Trunk, “Automatic pupillometry from digital images”, IEEE Transactions on Biomedical Engineering , 51(9): 1619-1627, September, 2004.