I Can See Clearly Now: The High-Tech Look of the Future!
Irving J. Arons
Ophthalmic Consulting Group
Arthur D. Little
Five or six years ago, Rob Webb of the Eye Research Institute in Boston (now the Retina Institute), invited me in to see a demonstration of a new device that he and others at the ERI had invented. It was the scanning laser ophthalmoscope. I can clearly recall the room full of instrumentation -- the large optical bench spilling over with beam splitters, mirrors, lenses and lasers -- and the TV monitor across the room from the slit lamp stand. Rob, his eyes dilated for the demonstration, sat down at the slit lamp and eased his chin into the support and we all watched in awe as first his cornea, and than other eye structures were clearly visible on the TV monitor as his eye approached the slit lamp objective. Finally, his retina was in full view, magnified on the screen, and by manipulation of a joy stick, could be panned.
This was my first exposure to the SLO, now a commercial diagnostic tool marketed by Rodenstock under license from ERI, and to diagnostic retinoscopy and confocal microscopy. (How they managed to get that work bench full of instrumentation into the small box is another story for another day.) Then, two years ago, at the Bausch & Lomb National Research Symposium, held in Boston that year, Dwight Cavanagh of Georgetown University Medical Center wowed his audience with a video taped demonstration of the passage into his cornea using a confocal microscope that he and his research team had built. The tape showed, in real time, a cellular level trip through his epithelium, through Bowman's membrane, into the stroma -- showing nerve tissue and keratocytes -- and finally, the fine structure of his endothelial cells. It was a remarkable demonstration, one I understand he has repeated at several other conferences with the same type of audience response.
These demonstrations opened my eyes to the possibilities of both retinal scanning and confocal microscopy. Since then, I have attempted to gain a better understanding of both these fascinating diagnostic techniques, and others like them, as I see these tools as the future for performing both corneal and retinal diagnoses in collaboration with both surgical and therapeutic treatment of these important eye structures.
In very basic terms, the SLO works like a TV, sweeping a small spot of low intensity laser light across the retina (or other target tissue) in a raster pattern. Light reflected back through the pupil is collected by a photomultiplier and converted into an electronic signal which feeds a video monitor. By combining the output of two incident laser beams, a high degree of contrast is achieved. The scientists at ERI have also developed a confocal version of the SLO. By combining the focus of the flying illumination spot with the focus of the detector array, i.e., conjugate foci or confocal focus, the imaging contrast is increased independent of the illuminating wavelength. This is accomplished in the confocal scanning laser ophthalmoscope by reusing the source optics for detection. The major advantage of this instrument is the crisp and complete retinal images formed using a low power HeNe laser source, without the need for dilating the pupil.
The confocal microscope, demonstrated by Dr. Cavanagh, is also a light scanning system, but employs an incoherent white light source focused through a scanning/rotating disc (Nipkow disc) made up of multiple pinholes arranged in an Archimedian spiral. Each spot of light is focused on a plane to rebuild the field visualized in the confocal image. By focusing the reflected light through the same pinhole but 180° out of phase (the Petran and Hadravsky or dual/tandem light path model), or through the same pinhole (the Kino single light path scheme), internal reflections are minimized and the resulting signal to noise ratio maximized. Thus, only the spot in the focal plane of both the illuminating source and the detector (through the pinhole) is in focus, and all other images are defocussed.
Over the past two years, several new instruments for performing diagnosis of various eye structures have been introduced. These include the scanning laser ophthalmoscopes from several firms; Carl Zeiss, Rodenstock, the now defunct Heidelberg Instruments, and developmental models from Allergan/Humphrey and this year from Kowa-Optimed. In addition, at this years AAO meeting (1990), Innovision demonstrated its retinal scanner/tracking device to be marketed this summer, and Nidek unveiled its 3-D camera which takes a 3-dimensional image of the retina captured on special film, that after processing, can be viewed without special glasses or a viewer, just by being held up to a white light source.
Yes, the times are changing and new diagnostic instruments are reaching the market that will make a difference in both diagnosing and treating eye disease. I will try and keep you apprised of these new technologies as they are introduced.