Optical Coherence TomographyAngiography (OCTA) as a New Modalityfor Posterior Segment Imaging

Abstract

Optical coherence tomography (OCT) is an imaging system designed to acquire high resolution cross-sectional retinal images. The technique is analogous to ultra¬sonography, but the tomographic OCT image is an intensity map of light back-scattered or reflected from tissue structures. Optical Coherence Tomography can essentially be thought of as a sort of " Ultrasound with light ".In ultrasound imaging ,sound echos are measured ,while OCT measures the echoes of back-scattered light after passing through a sample. In ultrasound, echoes are measured by measuring the time delay of agenerated soundwave .In OCT, because of the high speed of light, this is impractical-direct electronic measurement of time differences of mere femtoseconds are too complex, so an alternative method of measuring the optical echoes is used. OCT is based on a classical principle of optics, which is known as low-coherence interferometry. Low-coherence interferometry is a simple method, which can be used to precisely measure distances between objects by measuringLight reflected from them. Optical coherence tomography angiography (OCTA) is a new non-invasive imaging technique that employs motion contrast imaging to high-resolution volumetric blood flow information generating angiographic images in a matter of seconds. OCTA compares the decorrelation signal (differences in the backscattered OCT signal intensity or amplitude) between sequential OCT b-scans taken at precisely the same cross-section in order to construct a map of blood flow. Axial bulk motion from patient movement is eliminated so sites of motion between repeated OCT b-scans represent strictly erythrocyte movement in retinal blood vessels OCTA requires higher imaging speeds than most currently available OCT systems can provide in order to obtain a densely sampled volume. Conventional OCT device scanning speeds would result in too much trade-off between decreased field of view, lower image quality, and greatly increased scanning time. A split-spectrum amplitude decorrelation angiography (SSADA) algorithm use the natural flow as the target of the algorithm, and then,no need for injection of any dye to obtain the image of retinochoroidal vascular network. The flow is detected by comparison of same acquisitions in different times. SSADA algorithm is based on amplitude decorrelation. OCT became the gold standard for retinal disease during the last decade and took the most important place in multimodal exploration in retinal diseases. Time domain OCT (TD OCT) first, spectral domain OCT (SD OCT) and swept source OCT (SS OCT) now, are the best friends for the retinal daily practice of any ophthalmologists. OCT (SD and SS) ,permit an anatomic analysis of retinal tissue with now high resolution near to the pmeter, three dimensional imaging, the accuracy of these devices permit many clinical use in retinal pathology, glaucoma even in neuro ophthalmology. OCT angiography is a new method of retinal imaging based on high resolution analysis which permit to visualizate retinal and choroidal vessels. OCT angiography has slower imaging speeds and more limited retinal coverage than structural OCT. On the positive side, OCT angiography has the advantage that there is no dye injection required. Imaging can be performed in situations when conventional angiography is not indicated, repeated onevery patient visit, and dynamic changes can be assessed. Optical coherence tomography angiography cannot assess vascular permeability as one would with fluorescein or indocyanine green. Leakage as detected during angiography is a diffusion process, not a bulk flow phenomenon. The presence of leakage gives us physiologic information that is helpful in disease analysis. However, images obtained by OCT angiography are not obscured by leakage and therefore clearly show involved vessels. In addition, since OCT angiography is obtained from structural OCT data, it enables three-dimensional volumetric imaging of the retinal and choroidal vasculature.