Laser imaging of the retina

Abstract

The development of the scanning laser ophthalmoscope (SLO), first conceived by Webb et al in 1980 1 heralded the first widespread application of lasers for retinal imaging. A highly collimated beam from a laser is swept over the retina delivering all its energy to a very small spot for a very short time, typically of the order of some tens of nanoseconds. Light reflected from the spot is detected and synchronously decoded to form an image on a monitor.The use of a laser in this way oVers a number of fundamental advantages for retinal imaging: + as the input laser beam occupies only a small part of the pupil most of the pupil is available for light collection permitting non-mydriatic imaging and levels of illumination that are a factor of 200 lower than with the fundus camera + there is no need to employ high intensity flash so rapid imaging can be carried out oVering the potential for serial imaging + since imaging is carried out point by point, data are readily acquired as digital images allowing quantitative measurements and image processing + since the detected light is focused at a single point confocal imaging can be employed to give three dimensional images + the scattered light can be imaged separately from the reflected component, Tyndall imaging, to enhance pathological structures that have a high scattering component + the wavelength of the illuminating light can be selected by choice of laser so oVering the potential for multispectral imaging + changes in polarisation of the reflected light can be used to measure retinal nerve fibre layer (RNFL) thickness. While a number of these factors make imaging simpler there are three major advances that the SLO oVers to retinal imaging-the ability to do quantitative imaging, multispectral imaging, and three dimensional imaging. Quantitative imagingThe SLO is inherently a digital imaging technique; the image is produced by taking a series of samples of the light reflected from the retina as the beam is scanned in a raster fashion whereas with the fundus camera the whole of the retina is imaged simultaneously. However, the advantage of digital acquisition of data is now available with the digital fundus camera, so what advantages does laser imaging offer?One of the hopes of digital imaging is the automated analysis of retinal images both to identify abnormal pathology and to quantify the degree of pathology present.2 The precursor to image analysis is the need to correct for any non-uniformities in the illumination of the retina.3 In theory the SLO provides a uniform illumination of the retina so minimising the need for correction. In practice, however, there is still some non-uniformity in illumination.A second advantage is the ability to select the laser wavelength to improve the contrast of the features under analysis. This will be discussed later, although relatively little work has been done on it.But perhaps the main contribution is the ability of the SLO to image rapidly. Since there is no need to utilise flash, images can be taken rapidly, as one avoids t...

A reference standard for the measurement of macular oedema

“…These features combine to make it a very versatile instrument. 73 The principle of the tomographic SLO The SLO produces an image by rapid, two dimensional scanning of a laser spot across the retina. Any laser wavelength can be used, but red or infrared light is usually chosen for three dimensional imaging since the longer wavelengths penetrate further.…”

Digital Ocular Fundus Imaging: A Review

“…Each of these techniques can take advantage of SLO and CSLO techniques [170,171,172,173]. Instead of illuminating and imaging the entire area simultaneously, the SLO technique illuminates only a spot in the eye fundus.…”

New Frontiers in Retinal Imaging

“…One major advantage of SLO is its ability to perform confocal imaging [19,20] . Taking advantage of the principle of confocal microscopy, confocal SLO moves a confocal aperture between two end points to obtain a number of tomographic slices and to extract depth information [21,22] . Confocal SLO and OCT have become standard instruments for scanning the optic nerve head in glaucoma and are widely being used for imaging the RNFL [23,24] .…”