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Confocal Scanning Laser Ophthalmoscopy
Last reviewed: 23.04.2024
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[How Confocal Scanning Laser Ophthalmoscopy Works
The Heidelberg Retinal Tomograph (HRT; Heidelberg Engineering GmbH, Heidelberg, Germany) is the only currently available confocal scanning laser ophthalmoscope. This device uses a confocal scanning system based on the principle of point illumination and point registration. In this system, one point on the retina or optic nerve disk is illuminated only for a time, allowing light emanating from the lighted area to pass through the hole until scattered light and fabric surfaces that are out of focus are in time. Thus, zones that are not adjacent to the focal plane are not illuminated and not visible. This allows you to get high-contrast images. In addition, it is possible to obtain a layered (tomographic) image of the retina and the optic disc. In HRT, a 670 nm diode laser is used to scan and analyze the posterior segment of the eye. A three-dimensional image is obtained from a series of optical sections in successive focal planes from 16 to 64. Information is obtained in two images - topographical and mirror images. The topographic image consists of 256x256 or 384x384 pixel elements, each of which is an indicator of the height in the respective localizations. The optical resolution in the cross section is approximately 10 μm, whereas in the longitudinal size the resolution is about 300 μm. In modern clinical practice, three scans are made on each eye, then they are averaged, creating an average topographic image. The picture is obtained with an unexpanded pupil, but with mydriasis, image quality in patients with narrow pupils and cataracts is increased. Reproducibility is better with narrow pupils.
Restrictions
For measurements of confocal scanning laser ophthalmoscopy of the optic nerve disc, an orientation plane is needed for the calculation of many parameters: excavation zone, disc and excavation ratio, excavation volume, neuroretinal rim zone, volume, nerve fiber thickness of retina, cross-sectional area of retinal SNV. The approximate plane used by modern software may change with time, especially in patients with glaucoma under varying topography. This change can lead to inaccurate measurements. The user should determine the border of the optic nerve disc. The form of excavation, the volume of excavation below the surface, the average depth of excavation, the maximum depth and area of the disk - parameters that do not depend on the orientation plane. The mismatch between the patient's horizontal plane and the scanner is also a potential possibility of significant changes.