Images are acquired either in raster scans (parallel frames), radial scans across the optic nerve, or as concentric circles to measure peripapillary RNFL thickness in a 3.46-mm scan circle centered on the optic nerve head (ONH). Using a near-infrared super-luminescent diode light, SD-OCT acquires 26,000 to 85,000 axial scans per second for an axial resolution of about 5 μm and transverse optical resolution of about 14 μm. Compared to previous models of time-domain OCT technology, spectral domain OCT (SD-OCT) produces higher resolution images by utilizing spectrally separated detectors and taking Fourier transform of broad spectral information ( Fig. OCT noninvasively acquires a real-time image of the ophthalmic structures in optical cross section. (Courtesy of Dr Teresa Chen of Massachusetts Eye and Ear Infirmary.) (a) Healthy eyes and (Continued) (b) glaucomatous eyes. GDx scanning laser polarimetry of the optic nerves are shown. 3.2 GDx scanning laser polarimetry of normal and glaucoma eyes. (Courtesy of Dr Teresa Chen of Massachusetts Eye and Ear Infirmary.) Fig.
(a) A healthy eye and (Continued) (b) a glaucomatous eye. Heidelberg retinal tomograph (HRT) confocal images of the optic nerve are shown. 3.1 Heidelberg retina tomographs of normal and glaucoma eyes. These different commercial devices utilize similar OCT acquisition principles but vary in scanning protocols and segmentation algorithms (discussed in depth in Chapter 7). Commercially available OCT devices include Cirrus HD-OCT (Carl Zeiss Meditec, Dublin, CA), Spectralis (Heidelberg Engineering, Heidelberg, Germany), RTVue-100 (Optovue, Freemont, CA), 3D OCT-2000 (Topcon Medical Systems, Oakland, NJ), and RS-3000 Advance (Nikon, Tokyo, Japan). However, the detailed and precise imaging of the optic nerve possible with optical coherence tomography (OCT) is the most common computerized imaging of the optic nerve in current clinical practice. Similarly, the GDx scanning laser polarimeter estimated RNFL thickness by measuring its birefringence using polarized light ( Fig. Technologies such as the Heidelberg Retina Tomograph (HRT, Heidelberg Engineering, Heidelberg, Germany) emerged to provide quantitative information about the topography of the posterior fundus using confocal laser scanning microscopy ( Fig. Imaging modalities have been developed to aid ophthalmologists in optic disc assessment to monitor structural signs of glaucoma. Along the same lines, stereoscopic photographs of the optic nerve and monochromatic photographs of the RNFL are limited by only fair agreement in interpretation between glaucoma specialists and the lack of quantitative information. Careful stereoscopic examination of the optic nerve remains a hallmark in detection and management of glaucoma, but subjective interpretation and two-dimensional documentation can limit the ability to identify subtle changes. Visual field defects may not become apparent until 40% of retinal ganglion cells are lost, suggesting that perimetry alone may not capture early disease. Prompt recognition of early structural changes is critical to mitigate permanent vision loss from glaucoma.
Glaucoma is characterized by progressive loss of retinal ganglion cells that causes structural changes to the optic nerve and thinning of the retinal nerve fiber layer (RNFL) with subsequent corresponding visual field defects.
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