A confocal SLO uses a near-infrared diode laser (675 nm) beam that rapidly scans the posterior pole in a raster fashion—similar to the way in which a television creates an image on a monitor. The reflected light is detected by a confocal photodiode that is conjugate to the retinal plane, and the digitized image is stored in a computer. The confocal filter ensures that only light reflected from the narrow spot illuminated by the laser is recorded.
Stereoscopic high-contrast images can be produced with and without dyes such as fluorescein or ICG, and altering the laser wavelength permits selective examination of different tissue depths. The SLO is capable of imaging structures at very high magnification and high frame rate, which allows accurate diagnosis of retinal structures poorly seen by ordinary fundus cameras and does so using low levels of light exposure and improved contrast. In addition, a topographic 3-dimensional map with optical slices can be made digitally from 32 consecutive and equidistant optical section images obtained from the SLO. From this topographic map, retinal thickness can be estimated. Clinically, however, SLO has been used more in the objective evaluation of the surface contour of the optic nerve head in glaucoma than in the diagnosis of retinal disease.
One historic disadvantage of the SLO was the fact that it produced only a monochromatic image because a single-wavelength laser was used; however, true color representation of the fundus with an SLO is now possible by combining images taken using blue, green, and red lasers, as well as simultaneous ICG and FA by using an argon laser (488 nm) and a diode laser (795 nm) from an external source delivered by single-mode fibers. Present clinical applications include high-resolution ICG and FA, wide-field imaging of the retina through small pupils, microperimetry, and noninvasive assessment of retinal blood flow. In addition, SLO has also been combined with OCT in a single experimental unit that may offer even greater diagnostic capabilities in the future.
, BartschDU, MuellerAJ, BankerAS, WeinrebRN. Simultaneous indocyanine green and fluorescein angiography using a confocal scanning laser ophthalmoscope.1998;116:455–463.
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