Scanning laser ophthalmoscopy (SLO) is a method of examination of the

eye An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

. It uses the technique of

confocal laser scanning microscopy Confocal microscopy, most frequently confocal laser scanning microscopy (CLSM) or laser scanning confocal microscopy (LSCM), is an optical imaging technique for increasing optical resolution and contrast of a micrograph by means of using a s ...

for diagnostic imaging of the

retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...

cornea The cornea is the transparency (optics), transparent front part of the eyeball which covers the Iris (anatomy), iris, pupil, and Anterior chamber of eyeball, anterior chamber. Along with the anterior chamber and Lens (anatomy), lens, the cornea ...

of the human eye. As a method used to image the retina with a high degree of spatial sensitivity, it is helpful in the diagnosis of

glaucoma Glaucoma is a group of eye diseases that can lead to damage of the optic nerve. The optic nerve transmits visual information from the eye to the brain. Glaucoma may cause vision loss if left untreated. It has been called the "silent thief of ...

macular degeneration Macular degeneration, also known as age-related macular degeneration (AMD or ARMD), is a medical condition which may result in blurred vision, blurred or vision loss, no vision in the center of the visual field. Early on there are often no sym ...

, and other retinal disorders. It has further been combined with

adaptive optics Adaptive optics (AO) is a technique of precisely deforming a mirror in order to compensate for light distortion. It is used in Astronomy, astronomical telescopes and laser communication systems to remove the effects of Astronomical seeing, atmo ...

technology to provide sharper images of the retina."Roorda Lab"
— (last accessed: 9 December 2006)
Published on October 25, 2006—(last accessed: 9 December 2006)

Scanning laser ophthalmoscopy

SLO utilizes horizontal and vertical scanning mirrors to scan a specific region of the retina and create raster images viewable on a television monitor. While it is able to image the retina in real time, it has issues with reflections from eye astigmatism and the cornea. Eye movements additionally can confound the data from SLO.Webb RH, Hughes GW. "Scanning Laser Ophthalmoscope". ''IEEE Transactions on Biomedical Engineering''. 1981;BME-28(7):488-92.

Adaptive optics scanning laser ophthalmoscopy

Adaptive optics scanning laser ophthalmoscopy (AOSLO) is a technique used to measure living retinal cells. It utilizes adaptive optics to remove

optical aberration In optics, aberration is a property of optical systems, such as Lens (optics), lenses and mirrors, that causes the ''image'' created by the optical system to not be a faithful reproduction of the ''object'' being observed. Aberrations cause the i ...

s from images obtained from scanning laser ophthalmoscopy of the retina.

History

Scanning laser ophthalmoscopy developed as a method to view a distinct layer of the living eye at the microscopic level. The use of confocal methods to diminish extra light by focusing detected light through a small pinhole made possible the imaging of individual layers of the retina with greater distinction than ever before.Webb R, Hughes G, Delori F. "Confocal scanning laser ophthalmoscope". ''Applied Optics''. 1987;26(8):1492-9. However, using SLO for monitoring of individual retinal cells proved problematic because of optical aberrations created from the tissues of the anterior eye (specifically the

and

lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...

). These aberrations (caused additionally by astigmatism and other factors affecting eye position) diminished lateral resolution and proved difficult to remove. Adaptive optics was first attempted for SLO in the 1980s. This first attempt did not use wavefront-detecting technology with its deformable mirror and estimated aberrations through pre-measured factors such as astigmatism. However, this did not diffuse the small monochromatic aberrations resulting from light traveling through the anterior eye both into and out of the pupil during scanning. The invention and adaptation of the Shack–Hartmann wavefront sensor for the apparatus produced images of the retina with much higher lateral resolution.Liang J, Williams DR, Miller DT. "Supernormal vision and high-resolution retinal imaging through adaptive optics". ''J Opt Soc Am A.'' 1997;14(11):2884-92. The addition of microelectricalmechanical (MEMs) mirrors instead of larger, more expensive mirror deformable mirror systems to the apparatus made AOSLO further usable for a wider range of studies and for use in patients.

Procedure

The subject is placed in a dental impression mount fixed in a way to make it possible to manipulate the head in three dimensions. The subject's pupils are dilated using a dilating agent to minimize fluctuations from accommodation. After the eyes are sufficiently dilated, the subject is told to fixate on a target while in the mount.Roorda A. "Applications of adaptive optics scanning laser ophthalmoscopy". ''Optom Vis Sci.'' 2010 Apr;87(4):260-8. Once the subject is properly placed, wavefront correction and imaging takes place. A laser is collimated and then reflected off of a beam-splitting mirror. As in confocal SLO, light must pass through both a horizontal and a vertical scanning mirror before and after the eye is scanned to align the moving beam for eventual retinal faster images of the retina. Additionally, the light is reflected off of a deformable mirror before and after exposure to the eye to diffuse optical aberrations. The laser enters the eye through the pupil to illuminate the region it has been focused onto and light reflected back leaves the same way. Light returning from the mirrors passes through the first beam splitter onto another beam splitter where it is directed simultaneously toward a

photomultiplier tube Photomultiplier tubes (photomultipliers or PMTs for short) are extremely sensitive detectors of light in the ultraviolet, visible light, visible, and near-infrared ranges of the electromagnetic spectrum. They are members of the class of vacuum t ...

(PMT) and toward a Shack–Hartmann wavefront sensor array. The light going toward the photomultiplier is focused through a

confocal microscopy Confocal microscopy, most frequently confocal laser scanning microscopy (CLSM) or laser scanning confocal microscopy (LSCM), is an optical imaging technique for increasing optical resolution and contrast (vision), contrast of a micrograph by me ...

pinhole to remove light not reflecting off of the plane of interest and then recorded in the PMT. Light directed to the wavefront

sensor array A sensor array is a group of sensors, usually deployed in a certain geometry pattern, used for collecting and processing electromagnetic or acoustic signals. The advantage of using a sensor array over using a single sensor lies in the fact that an ...

is split up by the lenslets in the array and then recorded onto a

charge-coupled device A charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to a neighboring capacitor. CCD sensors are a ...

(CCD) camera for detection of optical aberrations. These aberrations are then subtracted from the images recorded in the PMT to vastly increase lateral resolution.

Applications

A major use of this increased lateral resolution from AOSLO has been the ability to determine the spatial distribution of cone cells around the fovea. Not only can the spatial density of these cells be found for a variety of regions within the retina, but the anisotropy of these cells can also be calculated to determine the axial orientation of retinal cells in the living subject. This represents a major benefit over typical histological examination of small numbers of donated human eyes.Chui TY, Song H, Burns SA. "Adaptive-optics imaging of human cone photoreceptor distribution." ''J Opt Soc Am A Opt Image Sci Vis.'' 2008 Dec;25(12):3021-9. AOSLO has also revealed significant decreases in foveal cone packing density for myopic eyes in comparison to emmetriopic eyes. This difference has been hypothesized to originate from a natural decrease in cone density with the increase in eye axial length associated with myopia. Abnormalities in photoreceptor structure in regions damaged by macular dystrophy have additionally been imaged by AOSLO. In these subjects, a dark area has been visualized within the macular lesion and morphologically abnormal photoreceptors have been visible on the lesion perimeter. Furthermore, scanning of subjects with cone dystrophy and retinitis pigmentosa (RP) has shown significant changes in cone packing density for these subjects compared to those with normal retinas. This presents a possible future use of AOSLO in phenotype tracking and confirmation for subjects with diseased genotypes. The imaging of

retinal pigment epithelium The pigmented layer of retina or retinal pigment epithelium (RPE) is the pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), ...

(RPE) cells in patients with and without retinal disease has also proved possible with the use of AOSLO. With the loss of photoreceptor cells, background scattered light decreases and the light focused on the RPE can be analyzed more clearly. As the loss of RPE cells represents the primary pathology of macular degeneration, this provides a possible future avenue for tracking RPE degradation ''in vivo''. This has been further proved with the analysis of

lipofuscin Lipofuscin is the name given to fine yellow-brown pigment Granule (cell biology), granules composed of lipid-containing residues of Lysosome, lysosomal digestion. It is considered to be one of the aging or "wear-and-tear" pigments, found in the l ...

granule autofluoresence in normal human and

rhesus macaque The rhesus macaque (''Macaca mulatta''), colloquially rhesus monkey, is a species of Old World monkey. There are between six and nine recognised subspecies split between two groups, the Chinese-derived and the Indian-derived. Generally brown or g ...

retinas by AOSLO. Comparison of this fluorescence in normal and diseased retinas with simultaneous imaging of cone structure and cone/retinal pigment cell ratio analysis has been shown to be possible and in the future may allow for the tracking of retinal damage from retinal dystrophies. AOSLO has already been used in rhesus macaques to track light damage to the macula from particular wavelengths. Additionally, AOSLO provides a greater degree of accuracy for eye tracking than possible before with other techniques. Because of the short scan time involved in AOSLO, eye motion itself represents an obstacle to taking images of the retina. Computational adjustments and modeling have been able to correct for aberrations caused by eye motion between frames. However, by tracking these aberrations based on changes to the retina between pictures, the effect of light on the individual orientation of the cone can be tracked. Research utilizing a visual stimulus and AOSLO eye tracking have yielded data on how the retina tracks movement at the microscopic level. The high degree of specificity and the ability to focus the laser on different levels of the eyes with AOSLO has additionally allowed for real time tracking of blood flow in the eye. By injecting fluorescin into macaques before scanning, fluorescence adaptive optics scanning laser ophthalmoscopy (FAOSLO) can be utilized to image individual capillaries in the nerve fiber layer and determine the thickness of the nerve fiber layer itself. Vessel pattern and diameter for these capillaries have been measured throughout the regions scanned by FAOSLO. This has future applications for monitoring glaucoma patients who either have changes in nerve fiber layer thickness or alterations in vasculature from damage to the retina.

Comparison to retinal dissection and other imaging techniques

AOSLO represents an advantageous alternative to retinal dissection for a variety of reasons. Analysis of cone packing density before AOSLO was only possible on mounted eyes from eye donor banks. As this method could not measure changes to cones in living eyes, it could not be used to track retinal changes over time or eye movements. With the use of living subjects, AOSLO allows for these measurements as well as easier control of age and other confounding factors while maintaining similar anatomical results for cone packing density. Future clinical implications for AOSLO are also possible. AOSLO compares favorably with other retinal imaging techniques as well.

Fluorescein angiography Fluorescein angiography (FA), fluorescent angiography (FAG), or fundus fluorescein angiography (FFA) is a technique for examining the circulation of the retina and choroid (parts of the fundus) using a fluorescent dye and a specialized camera. ...

uses injection of a fluorescein dye to image the back of the retina. It is a commonly used technique but it has a large number of side effects, including nausea in one fifth of patients and in some cases death from anaphylaxis.

Optical coherence tomography Optical coherence tomography (OCT) is a high-resolution imaging technique with most of its applications in medicine and biology. OCT uses coherent near-infrared light to obtain micrometer-level depth resolved images of biological tissue or oth ...

(OCT) represents a powerful clinical tool for monitoring retinal physiology in patients. OCT uses low coherence interferometry to differentiate tissues within the eye and create a cross section of a living patients’ retina non-invasively. It actually has greater axial resolution than AOSLO. However, AOSLO represents a method with much greater translational resolution than OCT and can thus be used to track minor lateral physical changes such as the effects of eye movements on the retina. A combination of AOSLO and OCT has recently been attempted in one apparatus to produce the first three-dimensional images of individual cone cells and illustrate the overall cone mosaic near the fovea at high speeds.Pircher M, Zawadzki RJ, Evans JW, Werner JS, Hitzenberger CK. "Simultaneous imaging of human cone mosaic with adaptive optics enhanced scanning laser ophthalmoscopy and high-speed transversal scanning optical coherence tomography". ''Opt Lett.'' 2008 Jan 1;33(1):22-4.

Notes

External links

Optos website
{{DEFAULTSORT:Scanning Laser Ophthalmoscopy Diagnostic ophthalmology Medical equipment