Abstract
Optical coherence tomography angiography (OCTA) based on mathematical processing of sequentially acquired structural OCT images has been applied widely in both retinal and choroidal research and may have advantages over traditional angiography. Images obtained by OCTA are rendered under the assumption that the only moving entity in the retina is blood flow. Optical phenomena and image processing algorithms may create imaging artefacts. Therefore, OCTA images require careful interpretation. This review discusses the dark signals seen in the choroidal vasculature on OCTA using multiple factor analysis. For accurate and comprehensive interpretation of the choroidal vasculature, we recommend simultaneous consideration of the laser light penetration depth and masking effect of retinal pigment epithelium, the orientation of vessels in relation to the scanning lasers and blood flow, the range of regional detectable velocity of blood flow, atrophic tissues in the periphery, and absorption of superior vessels on the scanning laser.
Highlights
Optical coherence tomography (OCT), including spectraldomain OCT (SD-OCT) and swept-source OCT (SS-OCT), can yield depth-resolved evaluation of reflectance data, together with a volume of three-dimensional information
OCT angiography (OCTA) using either SD-OCT or SSOCT assumes that the only moving entity in the retina is blood flow and visualises the vasculature based on motion contrast; moving tissue continuously produces OCTA signals, while stationary tissue produces nearly constant reflection or scattering [3]
It has been reported that SSOCTA is better able to demarcate the full extent of choroidal neovascularisation (CNV) than SD-OCTA [5]
Summary
Optical coherence tomography (OCT), including spectraldomain OCT (SD-OCT) and swept-source OCT (SS-OCT), can yield depth-resolved evaluation of reflectance data, together with a volume of three-dimensional information. Limiting factors include patient cooperation, the time-consuming nature of processing high-resolution images, projection artefacts due to superficial blood flow leading to difficulty in interpreting the networks of blood vessels within the deep retinal layers, and the bulk motion caused by circulating blood and any movement of tissue with respect to the OCT device [8]. The directional relationship between tissue or blood flow and the scanning laser contributes to the final strength of the signal. This is a common phenomenon in SD-OCT. If blood flow is parallel to the orientation of the scanning laser in the regions of the optic disc and the macula, the image becomes dark on both OCT and OCTA (Figure 2). Choroidal blood flow may appear to be dark, because its intensity does not reach the threshold of the current OCTA devices
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