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Color scanning laser ophthalmoscope provides true color imaging of fundus
Laser provides this advantage without increasing imaging time or level of exposure.
ABERDEEN, Scotland — According to a recent study conducted here, the color scanning laser ophthalmoscope (SLO) provides all of the advantages of the currently available monochromatic device. In addition, it has the added advantage of true color representation of the fundus without increasing imaging time or level of exposure.
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“The color SLO has added value over other digital imaging techniques as a diagnostic tool, with a capability to de tect and the po tential to quan tify a wide range of fundus pathology,” said Ayyakkannu Manivannan, PhD, one of the researchers.
Dr. Manivannan and colleagues at the University of Aberdeen examined the images from a custom-made color SLO of one healthy volunteer and five patients with a variety of fundus lesions.
“Low-power blue (argon, 488 nm), green (diode-pumped YAG, 532 nm) and red (semiconductor diode, 670 nm) lasers are combined using a fiber optic system to produce a single beam,” he said. Study results were recently published in the Archives of Ophthalmology.
All three lasers were modulated to pro duce an ultra-short pulse. Each laser switched on for 22 ns, and every point scanned on the retina was exposed to the three lasers sequentially. The total time for each image was 40 ms.
“As the lasers are switched sequentially, the total fundus exposure is 120 µW, which is the same as that from an individual laser,” he explained.
After the reflected light is decoded to extract the red, green and blue color information, the true color fundus image is shown live on a computer monitor.
SLO images of the healthy volunteer and the five patients were compared to digitized fundus camera photographs and/or digitally acquired red-free images. When comparing the SLO images to the digitized fundus photo graphs, the background fundus and retinal vasculature had a similar appearance. However, the SLO provided better quality information in patients with ocular histoplasmosis, macular dystrophy and optic nerve drusen.
“By operating the color SLO in the indirect mode, macular edema could be clearly seen as lines and ridges surrounding the fovea,” he said.
Resolution, patient comfort
According to Dr. Manivannan, fundus cameras have higher resolution and a larger field of view than most SLOs. When capturing a 45° field of view, a fundus camera provides a retinal resolution of 6 µm, while an SLO provides a resolution be tween 10 µm and 15 µm.
“Scanning laser ophthalmoscopic images have higher contrast due to the SLO imaging process, which involves illuminating only one point on the retina at a time, resulting in less scattered light. The relatively small depth of field of the SLO also reduces light scattered by deeper tissue layers,” he explained.
While the image detail provided by the SLO and the digitized fundus camera are similar, the SLO provides increased patient comfort due to the lower light levels. When comparing images captured by the SLO and the digitized fundus camera, exudates are a little bit paler with the SLO. However, vessels, exudates and pigmentary markings are all easily seen, and colors are as expected.
In addition, with the SLO image, the optic disc appears darker than it does with the fundus camera digitized image.
“This is mainly due to the use of a confocal aperture, which gives a depth resolution of approximately 600 µm. This results in light being collected only from a thin layer, improving detail but reducing intensity. In a structure such as the optic disc, its topography can be imaged further using the confocal optics,” he said.
Advantages of SLO
At the RPE level, more detail can be seen in the SLO images than in the fundus camera images. Another advantage of the SLO is that images are taken and stored digitally as three separate frames. This allows for significant data manipulation.
“Images are ready for automated analysis, eg, for detection of the number of microaneurysms, which may be of great benefit in screening programs for diabetic retinopathy. The drawbacks of SLOs are their smaller field of view, lower resolution and higher equipment cost when compared to fundus cameras,” he said.
For Your Information:Reference:
- Ayyakkannu Manivannan, PhD, can be reached at the department of biomedical physics and bioengineering, University of Aberdeen, Aberdeen AB25 2ZD, Scotland; +(44) 01224-554272; fax: +(44) 01224-685645; e-mail: mani@biomed.abdn.ac.uk.
- Manivannan A, Van der Hoek J, et al. Clinical investigation of a true color scanning laser ophthalmoscope. Arch Ophthalmol. 2001;119:819-824.