![]() Several studies have been performed to assess the diagnostic capability of SD-OCT in perimetric glaucoma. The calculated ONH parameters, except for disc area, are then compared to a normative database (see figure 2). This allows the 3.4 mm RNFL circle to always be centered in the same spot within the cube, which was not possible with TD-OCT. Cirrus does this by defining the edge of the disc as the termination of Bruch’s membrane and then finds the shortest perpendicular distance to ILM, AKA minimum band distance, to define the inner cup margin in each slice in a spiral around the optic disc cube data until a center is located. SD-OCT also automatically outlines the optic nerve head, optic cup, and disc borders similar to mental estimations by clinicians, but then also calculates more objective measurements such as optic disc area and neuroretinal rim area in addition to the classic clinician-subjective average and vertical cup-to-disc ratios. ![]() The TSNIT map displays RNFL thickness values by quadrants and clock hours, and the RNFL peaks give a sense of the anatomic distribution of nerve fiber axons represented by the superior and inferior bundles that emanate from the optic nerve (see figure 1). It is displayed as a false color scale with the thickness values referenced to a normative database. The Cirrus RNFL map represents a 6 x 6 mm cube of A-scan data centered over the optic nerve in which a 3.4 mm diameter circle of RNFL data is extracted to create what is referred to as the TSNIT map (temporal, superior, nasal, inferior, temporal). SD-OCT can directly measure and quantify RNFL thickness by calculating the area between the internal limiting membrane (ILM) and RNFL border (how the edge of the RNFL is determined and how blood vessels are handled are different among the machines, which do not have interchangeable measurement outputs). Glaucoma is a group of many conditions sharing a final common pathway characterized by accelerated death of retinal ganglion cells and their retinal nerve fiber layer (RNFL) axons resulting in characteristic visual field defects and corresponding optic nerve head anatomical changes. Our review focuses on the Cirrus HD-OCT in diagnosing glaucoma and glaucoma progression. Each machine has different glaucoma scan patterns, proprietary software segmentation algorithms, and display outputs. Ultrahigh speed swept source OCT, ultrahigh resolution OCT, polarization sensitive OCT, and adaptive optics OCT are all on the horizon.Ĭurrently, the most common four commercially available SD-OCT devices in the US are: Cirrus HD-OCT (Carl Zeiss Meditec, Dublin, CA, USA), RTVue-100 (Optovue Inc., Fremont, CA, USA), Spectralis OCT (Heidelberg Engineering, Heidelberg, Germany), and Topcon 3D-OCT 2000 (Topcon Corporation, Tokyo, Japan). Only four years later, several companies started to release the next generation technology, spectral-domain OCT (SD-OCT), AKA fourier-domain OCT, which improved upon TD-OCT by capturing more data in less time at a higher axial image resolution, around 5 µm. OCT became widely popular in 2002 with the release of Stratus OCT, a time-domain technology (TD-OCT) that was well-studied and validated for use in glaucoma and retina and went on to become a standard structural imaging test. Automated software segmentation algorithms are able to outline the retinal nerve fiber layer with much precision, which is relevant in glaucoma since this layer is thinned as ganglion cells are lost. Optical coherence tomography (OCT), first described in 1991, is a noncontact, noninvasive imaging technique that can reveal layers of the retina by looking at the interference patterns of reflected laser light. It will provide a brief review of all modalities, but focus primarily on the Cirrus machine. This is a review of the utility of spectral-domain optical coherence tomography in the diagnosis and management of glaucoma. ![]() ![]()
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