Abstract
Optical coherence tomography (OCT)-based angiography (OCTA) provides in vivo, three-dimensional vascular information by the use of flowing red blood cells as intrinsic contrast agents, enabling the visualization of functional vessel networks within microcirculatory tissue beds non-invasively, without a need of dye injection. Because of these attributes, OCTA has been rapidly translated to clinical ophthalmology within a short period of time in the development. Various OCTA algorithms have been developed to detect the functional micro-vasculatures in vivo by utilizing different components of OCT signals, including phase-signal-based OCTA, intensity-signal-based OCTA and complex-signal-based OCTA. All these algorithms have shown, in one way or another, their clinical values in revealing micro-vasculatures in biological tissues in vivo, identifying abnormal vascular networks or vessel impairment zones in retinal and skin pathologies, detecting vessel patterns and angiogenesis in eyes with age-related macular degeneration and in skin and brain with tumors, and monitoring responses to hypoxia in the brain tissue. The purpose of this paper is to provide a technical oriented overview of the OCTA developments and their potential pre-clinical and clinical applications, and to shed some lights on its future perspectives. Because of its clinical translation to ophthalmology, this review intentionally places a slightly more weight on ophthalmic OCT angiography.
Highlights
Optical coherence tomography (OCT) is a non-invasive and non-contact optical imaging technique [1, 2]
Observing two OCT signals – one is backscattered from surrounding biological tissue and the other one is backscattered from a flowing vessel – over time, the OCT signal backscattered from tissue components remains steady for there is no movement in the tissue, while the OCT signal backscattered from vessel changes over time as the red blood cells (RBC) tumbling and moving while flowing through the vessel, as shown in Fig. 2, a simplified schematic figure
In order to minimize the projection artifacts and reveal the actual vasculatures in the outer retinal avascular space (ORAS) for choroidal neovascularization analysis, Zhang et al [94] proposed a model to mimic the angiogram in the ORAS, which was defined as a space between the outer boundary of plexiform layer (OPL) to the Bruch’s membrane
Summary
Optical coherence tomography (OCT) is a non-invasive and non-contact optical imaging technique [1, 2]. With the improvements of scanning speed, image resolution, and SNR in FD-OCT, it allows more information from the biological tissues to be acquired and visualized in a relatively short amount of time, and makes the implementation of OCTA techniques become feasible. Doppler OCT shows improved velocity sensitivity using FD-OCT system, giving promising results in the detection of a range of velocities in phantom studies, its application to biological tissues in vivo, potentially clinical translation, is not very successful. This outcome may be due to several limitations. The phase difference is vulnerable to the sample motion, for example bulk motion or eye movements induced by saccades, and obstruct the acquisition of 3D imaging, for several seconds are often required for volumetric imaging
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