37 noted in 2010 that the B-scan rates for repeat scans needed to be fast enough such that bulk motion between B-scans was less than the OCT beam waist radius. In optimizing the method, Mariampillai et al. In their work, speckle variance was calculated as the variance of the OCT reflectance amplitude over three repeated B-scans at the same location. 36 extended the technique and presented speckle variance detection of microvasculature in a dorsal skinfold model using a swept-source OCT system in 2008.
34, 35 Specifically, the speckle pattern stays relatively constant over time for static objects while the pattern changes for objects in motion. Speckle arises as a property of the interferometric nature of OCT, and speckle variation contains information regarding the motion of scatterers. 33 adapted laser speckle analysis for time-domain OCT.
Optical coherence tomography angiography based on amplitude or intensity was initially described in 2005, when Barton et al. 1A), 10 it is not well suited for angiography of retinal and choroidal microvasculature, where vessels are nearly perpendicular to the OCT beam. 8, 9 Although Doppler OCT could measure and quantify blood velocity in larger vessels ( Fig. Doppler OCT uses the flow-induced Doppler phase shift between adjacent A-scans to calculate axial velocity. Since the early days of time-domain OCT, Doppler OCT has been explored as a tool for blood flow imaging. Because OCT systems are widely used in ophthalmology, its application to blood flow visualization and measurement could make clinical use more practical. 7 The limitations of these approaches include difficulty of use, poor reproducibility, large population variation in blood flow parameters, or limited availability of single-use instruments. Techniques such as ultrasound color Doppler imaging, laser Doppler velocimetry, laser speckle assessment, and blue field entopic technique have provided valuable insights into retinal physiology, but have not seen wide clinical use. 6įor more than half a century, scientists, engineers, and clinicians have collaborated to devise technologies to visualize and quantify changes in the retinal and choroidal vascular networks that supply the eye. Doppler OCT uses the Doppler phase shift to quantify blood flow in larger vessels and measure total retinal blood flow, 5 an application that we will not review, while OCTA is more concerned about separating moving scatters from static background tissue to create angiograms.
We want to emphasize that while Doppler OCT and certain implementations of OCTA both use phase information, the fundamental goals of Doppler OCT and OCTA are different. It will furthermore highlight the developments in post-processing and visualization tools that aid clinical interpretation. This review will provide a historic overview of OCTA techniques based on these principles of flow detection. These intrinsic contrasts can be broadly classified as Doppler shift and speckle variance/decorrelation. These techniques aim to contrast blood vessels from static tissue by assessing the change in the OCT signal caused by flowing blood cells. In order to develop a no-injection, dye-free method for visualizing ocular vasculature, a number of functional extensions of OCT has been explored.
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