OCT showing both macular edema and subretinal fluid in a diabetic patient The RPE/Bruch’s complex is one of the layers ascribed as zone as they are inseparable owing to interdigitation of cellular structure or tissue.The term zone describes those regions on OCT whose anatomical correlation is not clearly delineated.The term band refers to the three-dimensional structure of the retinal layers anatomically.Suggested the terms band, layer, and zone for the layers of the retina In brief, OCT angiography uses motion contrast by comparing the decorrelation signal between multiple B-scans obtained at each retinal cross-section to detect blood flow, employing the principle that theoretically only circulating erythrocytes within the retinal capillaries should be moving in the retina. Most recently, both spectral domain and swept source OCT have been used to generate non-invasive non-dye-based OCT angiography (OCTA) images. The enhanced axial resolution along with the faster scanning speeds, which allows for greater image averaging, improves image quality and the ability to visualize deeper structures in more detail. This wavelength provides an axial resolution of about 5.3 um in tissue compared to the approximately 5 um axial resolution of the standard 800 nm wavelength of commercial spectral domain devices. This technology uses longer wavelengths of 1050-1060 nm for deeper tissue penetration without the need for EDI. Swept-source technology, uses a wavelength-sweeping laser and dual-balanced photodetector, allowing for faster acquisition speeds of 100,000-400,000 A-scans per second. EDI employed the use of image averaging and it set the zero-delay line to adjacent to the choroid. In the late 2000s, the advent of enhanced depth imaging (EDI) allowed for better visualization of the choroid and choroidoscleral interface using the spectral domain system. Spectral-domain systems typically operate at 800-870 nm wavelengths, although longer wavelengths of 1050-1060 nm are being developed for deeper penetration in the tissue. This diminishes the chance of inadvertently missing pathology. Whereas most time domain OCTs image 6 radial slices, spectral domain systems continuously image a 6mm area. Whereas most time domain OCTs are accurate to 10-15 microns, newer spectral domain machines may approach 3-micron resolution. Spectral-domain systems increase the signal-noise ratio by image-averaging multiple B-scans at the same location. This increased scan rate and number diminishes the likelihood of motion artifacts, enhances the resolution, and decreases the chance of missing lesions. Spectral-domain technology, on the other hand, scans approximately 20,000-40,000 A-scans per second.
Because the slices are 30 degrees apart, care must be taken to avoid missing pathology between the slices. Time domain systems acquire approximately 400 A-scans per second using 6 radial slices oriented 30 degrees apart. Swept Sourceįrom its inception, OCT images were acquired in a time-domain fashion. the retina can be viewed through the pupil), OCT has gained wide popularity as an ophthalmic diagnostic tool. OCT was first introduced in 1991 by Huang and colleagues and has found many uses outside of ophthalmology, where it has been used to image certain non-transparent tissues. OCT utilizes a concept known as interferometry to create a cross-sectional map of the retina that is accurate to within at least 10-15 microns. Optical Coherence Tomography (OCT) is a non-invasive diagnostic technique that renders an in vivo cross-sectional view of the retina.
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