Scanning Laser Ophthalmoscopy
The confocal scanning laser ophthalmoscope (SLO) generates retinal images by scanning an illuminated spot on the retina in a raster pattern. It uses a Maxwellian view system to build the retinal image. Because the system is confocal, the scattered light can be rejected by the crystalline lens, as can the fluorescence, allowing for the use of shorter wavelengths in autofluorescence imaging. A photodiode is used to detect the light received from the eye. Because photodiodes are cheaper than high-resolution 2-dimensional sensors, some of the increased costs of the scanning mechanism are offset.
A variety of wavelengths can be used as dictated by need and the availability of a laser system. Infrared imaging can be used to evaluate the fundus; in some circumstances, it provides more comprehensive information than that revealed by color photography. For example, some types of pseudodrusen (subretinal drusenoid deposits) are not as prominent in color fundus photographs, but are easy to view in infrared images. Choroidal nevi reflect infrared light, consequently appearing bright in infrared imaging. The infrared image can be used to perform ocular tracking.
The chief factor that determines the resolution of SLOs is the size of the illuminated spot on the retina, which in turn is determined by the wavelength used, the numerical aperture of the illuminating system, any aberrations in the optical system, and scattering. SLOs scan the fundus with a dense pattern of spots. The resolution depends on the number of spots per unit area; a metric that is sometimes called the “digital resolution.” The digital resolution is invariably higher than the true optical resolution because the spots overlap.
In angiography, the dye is injected and then imaging begins. Some systems are able to obtain high-speed (eg, 15 frames per second) imaging of the dye’s passage through the eye. If the excitation laser and the barrier filter are put into place, no fluorescein dye is used, and the gain is turned up, an autofluorescent image of the fundus can be obtained. This method uses a blue-green wavelength that is absorbed by macular pigment. The field of view of a scanning laser imaging system is determined by its mechanical scanning system as well as the optics used to image the eye. The posterior pole can be imaged using various scan parameters. With wider-angle lens attachments, up to 55°, and in some cases 110°, can be visualized.
A second approach to scanning laser ophthalmoscopy uses an ellipsoidal mirror. Rays emanating from one focus point on an ellipse will converge on the second focus point. If ellipse is rotated along an axis, an ellipsoidal surface can be generated. If this surface is then made into a mirror, rays passing through one point will converge on the second point. If one point is placed at the entrance pupil of the eye, light rays emanating from a large solid angle will converge on the second focus point of the ellipsoidal mirror to form an image. Commercial systems that use this principle are able to obtain images approximately 200° wide. With ocular steering, nearly all of the retina can be imaged. In this method, 2 lasers are used to obtain a pseudocolor-scaled image: a red laser and a green laser. Angiography can be performed with the use of the appropriate excitation lasers. The number of points imaged is large, and these systems do not record video-rate angiography. The systems that employ an ellipsoidal mirror have limited confocality; a green laser needs to be used in order obtain autofluorescent images.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.