TechnologyAs with any specialized area of study, ophthalmology has its own complex array of terminology, machinery, and processes. This section of the encyclopedia gives insight into some of the "tools of the trade", describing most of the unique technologies you might encounter if you spent a lot of time at your ophthalmologist's office.
Optical Coherence Tomography (OCT)Optical Coherence Tomography (OCT) is, perhaps, the most promising technology available to produce images of the eye. During the simple procedure, a laser quickly scans the dilated eye. Unlike other imaging methods, such as ICG and FA, which produce top views of the retina, the image produced is a cross-section of the layers of the retina. Each of the thin layers of photoreceptors, nerve cells, and pigment epithelial cells are clearly delineated.
A high-quality image will show where abnormalities have occurred, such as the formation of drusen or new blood vessels, or the detachment or breakage of any of the retinal layers. OCT is an important tool for the diagnosis and treatment of many retinal conditions.
Below is an OCT showing a cross-section of the retina, revealing pockets of fluid (the dark areas "sandwiched" between the green layers of retina) under the surface.
Indocyanine Green (ICG) AngiographyIndocyanine Green (ICG) Imaging is similar to fluorescein angiography (FA). Like FA, it is a useful tool for the diagnosis and treatment of several forms of macular degeneration and retinopathy. It is more effective than FA in producing images of blood vessel growth and damage in the deeper choroid layer behind the retina. It is also able to produce a better image than FA if blood is present in the macula.
The process is generally safe, with a minimum of discomfort, and is usually done in the doctor’s office. It involves dilating the eyes being photographed, and an injection of the green dye, usually in the arm, which circulates throughout the body, including the choroidal layer of the eye. A flash illuminates the eye and the resulting images are stored on a digital camera system. The fluorescent light that the indocyanine green dye emits can help to identify abnormalities in the circulation of the choroid. A series of images, or a movie of the dye moving through the vessels of the choroid, will help the ophthalmologist to decide upon the best treatment to prevent further vision loss.
Most ophthalmologists consider ICG angiography to be an adjunctive or secondary test which adds further information to the clinical picture, and is often done in addition to a fluorescein angiogram.
Below is an example of a typical Indocyanine Green angiogram, revealing an area of leakage (the bright area) just outside the macula in the center of the image. The optic nerve can be seen to the left.
Fluorescein Angiography (FA)Fluorescein angiography (FA) is a technology that allows photographs of the blood vessels of the retina to be taken. It is a useful tool for the diagnosis and treatment of several forms of macular degeneration and retinopathy as well as other disorders.
The process involves injecting the dye into a vein in the arm, just after the eyes have been dilated with eye drops. The yellow dye will travel throughout the bloodstream, including the vessels of the retina. It will fluoresce, or emit light, when an extremely bright light is flashed into the eyes. A series of photographs will show the entrance and exit of the dye through the blood vessels of the retina and just beneath it. Any areas of abnormal new blood vessel growth, leakage, or blockage, will appear darker or brighter than normal angiograms of the vascular retinal area.
The process is generally safe, with a minimum of discomfort, and is usually done in the doctor’s office. It is an important tool, because it will reveal the location of damage by leaking or pooled blood and will show where new blood vessel growth is occurring. This will help the ophthalmologist to decide upon the best treatment to prevent further vision loss.
Below is a typical flourescein angiogram showing an eye with leaking vessels (the bright area) in the middle of the macula. The optic nerve is seen to the right.
Amsler GridThe Amsler grid is a simple square chart with a dot in the middle of what resembles graph paper, and can be used to test ones central visual field while at home. The lines are usually black, and the background white, though sometimes the colors are reversed. The test is a diagnostic tool that aids in the detection of visual disturbances caused by changes in the retina, particularly the macula (e.g. macular degeneration, Epiretinal membrane), as well as the optic nerve and the visual pathway to the brain.
When taking the test, the patient will be asked to look at the grid and concentrate on the dot in the middle, using one eye at a time. If the lines near the dot appear wavy, blurred, or are missing, macular degeneration, and the loss of central vision, may be occurring. The distorted image of the grid is not an optical illusion but is evidence that some photosensitive cells in the macula may have been destroyed, obscured (usually by blood) or displaced (as in a retinal detachment).