Scanning Laser Ophthalmoscopy
Timothy J. Bennett, CRA, OCT-C, FOPS
Penn State Hershey Eye Center
The confocal scanning laser ophthalmoscope (cSLO) is an instrument that can be used for several retinal imaging modalities including fluorescein angiography, ICG angiography and fundus autofluorescence. Some manufacturers have modified these instruments so that they are also capable of performing OCT. They are not however, capable of capturing full color images of the retina. Monochromatic laser illumination combined with a confocal optical system produce high-contrast, finely detailed images. This feature is due to the fact that illumination is provided solely from the laser beam and the remainder of the patient's pupil is available for light collection. The laser also delivers a very narrow wavelength band for more efficient excitation of fluorescence than the filtered flash illumination of the fundus camera.
A laser beam scans across the fundus in a raster pattern to illuminate successive elements of the retina, point-by-point at speeds up to 24 milliseconds. A confocal aperture is positioned in front of the image detector at a focal plane conjugate to the retina. The aperture blocks non image-forming light to minimize scatter and chromatic aberration. The light reflected from each retinal point is captured by a photomultiplier. The output of the photomultiplier is recorded and displayed in a digital video format. Unlike the traditional fundus camera, the cSLO has no optical viewfinder, eliminating the challenges associated with aerial-image focusing.
Confocal imaging reduces the effects of short wavelength scatter in the ocular media and confounding AF from crystalline lens. Left: Fluorescein angiogram with fundus camera. Right: Same eye photographed with cSLO.
The plane of focus effects reflectivity and appearance of retinal tissues based on depth due to the confocal aperture. All three images are at the same wavelngth. Focus is on the elevated optic disc on the left image. Tonality changes as focus is shifted to the retinal surface.
The laser scan rate is synchronized at a frame rate compatible with digital video display, providing a continuous high-speed representation of the flow dynamics of the retina and choroid during angiography. This can be especially useful when documentation of the very early filling stages is necessary, such as in identification of CNV feeder vessels and RAP lesions. Using SLO to perform fluorescein angiography allows for the measurement of capillary flow velocity. Flow velocity is determined by measuring the transit time of blood between two points on the video monitor that are separated by a known distance. Simultaneous fluorescein and ICG angiography can be performed at half resolution with a single injection of both dyes.
Images obtained with a cSLO are typically rectangular due to the raster scanning pattern. The standard angle of view varies by instrument with 15o, 20o, 30o settings being common. Both 35o and 55o auxiliary lenses are available for the Heidelberg HRA. The wide-angle lens is primarily used for peripheral imaging in either diabetic retinopathy or venous occlusive disease. The Staurenghi contact lens provides an angle of 150o with the Heidelberg HRA. In addition, on-the-fly montage software allows the operator to view a composite of the peripheral images as a single ultra-wide angle image.
The Optos optomap® fa offers angiographic capabilities in an ultra wide field format of 200o. The Nidek F-10 utilizes a variety of apertures to acieve different imaging effects.
High aquisition speed and eye-tracking software enables image sampling of several frames to produce a mean image. Sampling reduces noise and improves exposure.
cSLO technology can be used for quantification of retinal thickness or optic nerve head topography. Sequential cSLO scans captured at increasing depths can be combined to create three-dimensional topographic images of the retina or optic disc. Image stacks are aligned to create a final composite image to provide retinal thickness measurements of the macula or optic nerve head topography. Eye tracking technology and fast acquisition times minimize the effect of eye movement during imaging. Image alignment also facilitates generation of a mean image to reduce noise in fundus autofluorescence (FAF) images.