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Glaucoma causes an enlargement of the depression (or cupping) of the optic nerve head (or optic disc) where the optic nerve enters the eye. It also causes a thinning of the nerve fiber layer that radiates from the optic nerve head into the inner layer of the retina (the image-sensing layer that lines the back of the eye). These changes can be determined by eye doctors either by direct examination or with the use of specialized photography. However, a direct examination is subjective and can only provide a qualitative assessment that varies from doctor to doctor.
There are several new imaging technologies available that can measure the degree of cupping and retinal nerve fiber layer loss. Measurements derived from the images of a subjectís eye can be compared with a database of measurements from normal eyes to determine whether an individual has glaucoma. These measurements can then be tracked over time to determine whether the glaucoma is causing progressive damage. These objective and precise measurements are becoming more and more important to the management of glaucoma.
The AIG study utilizes the three most important and currently-employed glaucoma imaging technologies: optical coherence tomography (OCT), scanning laser polarimetry (SLP), and scanning laser tomography (SLT).
Optical Coherence Tomography (OCT)
Optical coherence tomography produces a cross-sectional image of the eye in a manner similar to ultrasound, except that it uses light instead of sound waves and can produce a much more detailed picture. This technology is used to evaluate both the optic nerve and the retina. The high-depth resolution of OCT provides very accurate measurements of the contour of the optic nerve head, as well as of the thickness of the retinal nerve fiber layer and other retinal layers affected by glaucoma.
Compared to time domain OCT, fourier domain OCT is 50 to 100 times faster. It has higher resolution and denser sampling than time domain OCT which means it is a better diagnostic tool in tracking glaucomatous thinning and other structural changes caused by retinal diseases.
We use the RTVue system to map structures such as retinal layers, more specifically the ganglion cell complex (gcc) which is preferentially affected by glaucoma. We can also map the peripapillary nerve fiber layer and get a 3d view of the nerve head.
We also use an ultrahigh speed swept source oct system, custom built by MIT, that scans 4 to 10 times faster than the RTVue and uses a longer wavelength which allows us to view deeper optic nerve head structures. The higher speed allows for wide field complete 3d volumetric mapping of the nerve fiber layer and ganglion cell complex.
We use both the RTVue and ss oct system to measure arterial, venous, and choroidal blood flow. Doppler OCT, which combines standard structural imaging with simultaneous flow velocity measurements, can measure functional changes caused by retinal diseases such as glaucoma. A pilot study has identified a significant correlation between decreased retinal blood flow and visual field defects in glaucoma and is independent of other indicators in measuring the progression of glaucoma. We hope that studying blood flow will be helpful in diagnosing glaucoma and assessing which patients are at risk for losing vision.
Scanning Laser Polarimetry (SLP)
Scanning laser polarimetry measures how retinal nerve fibers around the optic disc affect the polarization of laser light reflected from the eye.
Scanning Laser Tomography (SLT)
Scanning laser tomography measures the surface contour of the optic nerve head.
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