A team of ophthalmologists at the University of Aberdeen has developed a new technique that allows them to obtain full colour images of the back of the eye.
The new technique, developed by Dr Ayyakkannu Manivannan and his colleagues, uses low-power coloured lasers to give a full colour picture that ophthalmologists can use to recognise different diseases of the eye, such as those associated with diabetic eye disease. This is the most common cause of blindness in the working age population.
The research will be published today (Wednesday, December 19) in the Institute of Physics journal Physiological Measurement.
The most common way of taking a picture of the retina is to use a flash camera that needs a high light intensity to illuminate the retina. The retina is illuminated through the outer part of the dilated pupil, while the reflected light from the retina is seen through the central pupil. Unfortunately, this reduces the amount of light that can be detected by a factor of 16 and limits the image quality.
An alternative technique is to use the scanning laser ophthalmoscope (SLO). The SLO uses a narrow laser beam to illuminate the retina. As the beam only occupies a very small part of the pupil, the reflected light passes back through a large exit pupil reducing the intensity of light needed to take the picture. Although the lower light intensity is better for the comfort of the patient, this technique can only produce images in one colour, reducing the detail of the picture compared with a standard optical camera. Using coloured lasers simultaneously could produce a more detailed image, but would also increase the exposure of the retina.
Fortunately, Manivannan and colleagues have overcome this problem by designing and building an SLO with three low-power lasers that pulse rapidly in sequence, rather than simultaneously. The three lasers are
chopped to produce ultra-short pulses of light lasting for just twenty-two billionths of a second. A fibre optic system then combines the three pulses to produce a single three-coloured beam. Every point on the retina is exposed to pulses from the three lasers in sequence.
The new instrument uses the same amount of light as a conventional one-colour SLO, but the image formed is clearer because of the extra colour information. Twenty-five images are taken every second to avoid the image being spoilt by eye movements, making the total imaging time to be just 40 milliseconds.
“The colour information makes it easier to recognise different retinal diseases, such as those associated with diabetic eye disease, the most common cause of blindness in the working age population," said Dr Manivannan.
The authors wish to acknowledge The Wellcome Trust, Aberdeen University Research and Industrial Services Limited (AURIS) and Grampian Enterprise Limited (GEL).
The University of Aberdeen’s media information can be viewed online: http://www.abdn.ac.uk/central/news/univ/releases.hti