Imaging of the Eye

Imaging of the Eye

Eye Surface Profiler

The Eye Surface Profiler (ESP) is a new clinical instrument that is capable of measuring the anterior eye’s corneal and scleral topography. With the use of our customized software, the ESP provides axial radius maps of the ocular surface out to an 18 mm diameter. These topography maps can be used to estimate the surface area of tissue affected by a variety of lesions. We have also modified the instrument to improve focusing and observation of the fluorescein coverage during image acquisition.

 

A) live view of fluorescence for ‘complete (left)/ incomplete (right)’ tear film B) live view of ESP left fringe pattern C) live view of right fringe pattern D) ESP ‘source’ image E) ESP tangent angle data

 

High Speed Filming

High speed filming can be used to gain a better understanding of the dynamic movements of the eye and eyelids. Contact lens centration and movement is influenced by these dynamic movements during blinking. The human eye naturally blinks about 12 times per minute, each blink lasting about ¼ of a second (250 msec). The upper eyelid moves down and slightly inward during the downward phase of the blink, whereas the lower eyelid makes a small movement towards the nose during the blink. The eye naturally retracts during the blink and in the example, a puff of air strikes the cornea, causing a retraction of the eye during the reflex protective blink.

 

 

 

Simultaneous Thermal and Fluorescein Imaging of the Tear Film

The stability of the tear film can be assessed by a variety of non-invasive measurement techniques. One of these methods we are currently investigating is the use of thermal imaging at the same time as imaging the fluorescence of fluorescein dye in the tear film. As the tears become unstable and break-up, the fluorescein and thermal image show dark areas as the tears thin and become cooler.

 

Eye Tracking

The Tobii Glasses 2 eye tracking system is designed to be used for research in order to examine an individual’s eye movement behaviour. It includes the lightweight Tobii Glasses Head Unit with 4-infrared cameras monitoring the eyes and a forward-facing wide angle high definition scene camera to record the visual environment. The gaze sampling frequency is 100 Hz. The gyroscope and accelerometer built into the Glasses 2 eye tracker also provide angular velocity data for three dimensional head movements.

 

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. The example in the video shows the dramatic effect that the force of the eyelids can have on the topography of the cornea. At five 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).

 

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 secs 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 characterisitics of tear stability, tear flow, tear film break-up, and in assessing clinical problems such as dry eye and contact lens surface dryness.

 

 

The video above shows the gradual decay in non-invasive objectively measured tear film surface quality following a blink using videokeratoscopy.


Further Reading