Heidelberg Spectralis Optical Coherence Tomography Angiography: Technical Aspects.

Optical coherence tomography angiography (OCT-A) is a promising new method for visualizing the retinal vasculature and choroidal vascular layers in the macular area and provides depth-resolved functional information on blood flow in these vessels. OCT-A is based on the concept that in a static eye the only moving structure in the fundus of the eye is blood flowing through the vessels. Contrast is generated based on the difference between moving cells in the vasculature and the static surrounding tissue. Artifacts can arise due to scan positioning errors caused by normal ocular microsaccades. In order to avoid artifacts, a sequence of OCT B-scans in the exact same retinal location must be taken to detect flow. Active eye-tracking (TruTrack™) using the simultaneous acquisition of fundus and optical coherence tomography (OCT) images presents a very reliable method of acquiring OCT volume scans without motion artifacts and helps significantly improve signal-to-noise ratio. This system also allows the use of a full spectrum amplitude decorrelation algorithm that produces clear differentiation between blood flow and static tissue without sacrificing the axial resolution of OCT images. Accuracy in layer segmentation, which requires high-resolution OCT B-scans, is crucial for producing reliable OCT-A images. This can be achieved through automated or manual layer segmentation. During OCT scan acquisition, the effect of axial motion (e.g. a patient moving towards the camera) is compensated for by geometric alignment of successive B-scans before analyzing temporal changes.