George W. Hughes

Confocal scanning laser ophthalmoscope

A confocal scanning imager moves an illumination spot over the object and a (virtual) detector synchronously over the image. In the confocal scanning laser ophthalmoscope this is accomplished by reusing the source optics for detection. The common optical elements are all mirrors-either flat or spherical-and the scanners are positioned to compensate astigmatism due to mirror tilt. The source beam aperture at the horizontal scanner is small. Light returning from the eye is processed by the same elements, but now the polygons facet is overfilled. A solid-state detector may be at either a pupillary or retinal conjugate plane in the descanned beam and still have proper throughput matching. Our 1-mm avalanche photodiode at a pupillary plane is preceded by interchangeable stops at an image (retinal) plane. Not only can we reject scattered light to a degree unusual for viewing the retina, but we choose selectively among direct and scattered components of the light returning from the eye. One (of many) consequences is that this ophthalmoscope gives crisp and complete retinal images in He-Ne light without dilation of the pupil.

Scanning Laser Ophthalmoscope

An instrument is described which functions as a low light level ocular fundus camera and ophthalmoscope, and which is capable of making a wide range of quantitative measurements in the eye. Light levels for ophthalmoscopy (20 ¿W/cm2 at the retina) are at least two orders of magnitude below those in current use. A focused laser bearn forms a flying spot, moved physically by scanning mirrors. This allows a 20 ¿m or smaller resolution element, with only a 0.9 mm diameter area of the patients pupil used for the entering beam. The remaining pupillary area forms the exit pupil¿a critical inversion of the division of pupils necessary for systems using conventional imaging. It is this inversion which allows the low light level and the unique measurement capabilities. We discuss the constraints imposed by viewing the inside of a spheroid through a small hole in its wall, and our solutions¿both optical and electronic¿to these problems. We also describe electronic problems encountered in the video system, which arise from our special detection and display systems. Some specifics of the instruments measurement capabilities are discussed.

Scanning Laser Ophthalmoscopy: Clinical Applications

The scanning laser ophthalmoscope (SLO) provides a high-quality television image of the retina using less than 1/1000 of the light required for conventional indirect ophthalmoscopy. The SLO employs a new ophthalmoscopic principle in which a dim laser beam scans across the fundus, and light is collected only from one retinal point at a time. Since the instrument is highly light efficient, illumination levels are comfortable for the patient, and fluorescein angiography can be performed with one tenth of the usual fluorescein dose. Since a continuous, large depth of field view is displayed on the SLO screen and stored on video tape, repeated dynamic inspection of the vitreous, retina and vitreoretinal interface is afforded. In addition, any graphical material that can be displayed on a microcomputer monitor (such as text of video games) can also be impressed on the retinal pattern formed by the sweeping laser beam. The graphical material is thus observed directly by the patient and on the patients retina by the clinician. Since the exact retinal locus of each point in the graphical material is viewed directly, it is possible to perform perimetry directly on the retina, to measure acuity at arbitrary retinal loci, to study how patients with macular disease use residual functional retina for reading, and to perform distortometry with a retinal (Amsler-type) grid.

Reflectometry with a scanning laser ophthalmoscope.

We describe noninvasive techniques to optimize reflectometry measurements, particularly retinal densitometry, which measures the photopigment density difference. With these techniques unwanted scattered light is greatly reduced, and the retina is visualized during measurements. Thus results may be compared for each retinal location, and visible artifacts are minimized. The density difference measurements of the cone photopigment depend on the optical configuration of the apparatus. The cone photopigment density difference is greatest near the fovea and for most observers decreases rapidly with eccentricity. A research version for reflectometry and psychophysics of the scanning laser ophthalmoscope is described.

Fundus tracking with the scanning laser ophthalmoscope

A technique is described of using the video output of the scanning laser ophthalmoscope to monitor the positions of fundus features with respect to an input laser raster. The monitoring performance characteristics are discussed as well as tracking methods and possible applications in psychophysics and laser photocoagulation.

Anterior Segment Fluorescein Videoangiography with a Scanning Angiographic Microscope

The scanning laser ophthalmoscope can be modified to operate as a scanning laser biomicroscope for use in anterior segment fluorescein angiography. The substantial depth of focus, large field of view, co-axial illumination, low light levels, real-time television operation, and videorecording with immediate recall provide advantages not available with conventional photographic methods. Video techniques give a resolution slightly inferior to photography, but this is unlikely to be significant in clinical practice. A technique of traversing the entire anterior episcleral vasculature has been developed to give a comprehensive and reproducible angiographic record. Previous fluorescein studies suggesting the primary importance of retrograde (centrifugal) flow in the perforating anterior ciliary arteries were not supported; methodologic explanations are advanced. Several principles are proposed to improve techniques of anterior segment angiography.

Detectors for scanning video imagers

Detectors for scanning video (10-MHz) imagers should be chosen for their high quantum efficiency and internal gain. Because of the high bandwidth both photomultiplier tubes and avalanche photodiodes are limited by photon noise, so that dark noise is not the determining quantity.