Scanning Laser Ophthalmoscope
The scanning laser ophthalmoscope functions as both an ophthalmoscope and a fundus camera but requires significantly less light than those conventional flood illumination systems. This is because in the scanning laser ophthalmoscope, the use of a rapidly scanning laser (eg, a 670-nm diode laser) illuminating only a small spot of retina, allows inversion of the allocation of illumination and viewing apertures used in conventional systems. In other words, unlike the ophthalmoscope or fundus camera, where illumination uses most of the pupillary area with a separate small area reserved for viewing (Fig 8-22A), the scanning laser ophthalmoscope uses the larger area for light collection (“viewing”) and the smaller one for illumination (Fig 8-22B). It is this inversion that improves the optical collection efficiency, allowing lower light intensity levels to be used.
In a scanning laser ophthalmoscope, a highly collimated laser beam is physically moved via scanning mirrors over the retina in a grid pattern, delivering all its energy to a very small spot for a very short time. Light returned from each point is detected and synchronously decoded to produce an ocular fundus image.
Figure 8-22 Illumination and observation path in conventional ophthalmoscopy and fundus imaging versus scanning laser ophthalmoscopy. Unlike the ophthalmoscope or fundus camera, where illumination uses most of the pupillary area with a separate small area reserved for observation (A), the scanning laser ophthalmoscope uses the larger pupillary area for observation and the smaller one for illumination (B).
(Illustration modified by Kristina Irsch, PhD.)
The following sections describe a few types of scanning laser ophthalmoscopes.
Confocal scanning laser ophthalmoscope
In confocal scanning laser ophthalmoscopy, a pinhole is placed in front of the detector to cut off scattered or defocused light coming from outside the point of illumination or interest, which otherwise can blur the image (Fig 8-23). This results in a focused, high-contrast image of a single tissue layer located at the focal plane.
The position of the confocal aperture determines from which layer in the fundus the reflected light is collected, and it enables tomographic information to be extracted. More precisely, by moving the plane of the pinhole, multiple optical sections through the tissue of interest can be acquired.
The series of optical section images forms a layered 3-dimensional image and can be used to construct topography and reflectance images of the fundus. The topography image is constructed by identifying the peak intensity (reflectance) along the z-axis from all optical sections at each pixel location. The reflectance image is constructed as a summation of intensities along the z-axis from all of the optical sections at each pixel location.
Confocal scanning laser microscope
With the use of objective lenses, usually put in contact with the patient’s corneal surface, the confocal scanning laser ophthalmoscope can be turned into a confocal laser scanning microscope, enabling tomography through different depths of the cornea at cellular resolution (see BCSC Section 8, External Disease and Cornea).
Angiography and autofluorescence imaging
The use of various wavelengths allows for additional applications with the confocal scanning laser ophthalmoscope, such as fluorescein angiography, indocyanine green angiography, and autofluorescence imaging (see also BCSC Section 12, Retina and Vitreous).
Figure 8-23 Principle of confocal laser scanning ophthalmoscopy. A pinhole aperture prevents defocused or scattered light coming from outside the focal plane, which otherwise can blur the image, from entering the detector.
(Illustration modified by Kristina Irsch, PhD.)
For fluorescein angiography with the scanning laser ophthalmoscope, a blue argon laser at 488 nm is used to excite the dye, while a barrier filter at about 500 nm edge wavelength separates the excitation and fluorescent light.
The same laser is also used to generate fundus autofluorescence images, relying on the natural fluorescence occurring from the retinal layers, such as lipofuscin that accumulates in the retinal pigment epithelium. It may also be used (but without the use of the barrier filter) to create “red-free” or blue reflectance images, which can aid in the visualization of pathologies that have low contrast to the red color.
For indocyanine green angiography, on the other hand, a diode laser at 790 nm wavelength is used to excite the dye, while a barrier filter at 810 nm edge wavelength separates excitation and fluorescent light.
Scanning laser polarimeter
The retinal nerve fiber layer is birefringent, which means that polarized light travels through it at different speeds depending on whether the polarization is along or across the fibers (see Chapter 2). The scanning laser polarimeter is a confocal scanning laser ophthalmoscope with an integrated ellipsometer that enables the measurement of this retardation. There is a linear relationship between the thickness of a birefringent medium and its retardation. By measuring the total retardation of the human retina point by point in a raster pattern, from the change in polarization state in the light retroreflected from the fundus, a “topographic map” of the birefringent nerve fiber layer thickness in the eye’s retina can be created. This provides a quantitative method for detecting evidence of eye diseases such as glaucoma, which is characterized by loss of nerve fibers in its early state.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.