Introduction to OCT Angiography
What is OCT Angiography?
OCT Angiography (OCTA) is a new, non-invasive imaging technology through which microvasculature structures of the retina and choroid are visualized without the need for dye injection. Through advanced software, OCTA provides a snapshot of red blood cell movement at a fixed point in time allowing for a detailed presentation of vascular flow information at a specific vessel location. In general, dark areas on the scan represent no or low blood flow while bright areas indicate the detection of red blood cell movement. OCTA is a useful tool in not only clinically managing patients who have an underlying vascular disorder or are at risk for vascular disorders (such as patients with diabetes, hypertension, or age-related macular degeneration (AMD)) but can provide further information on the pathophysiology of these vascular disorders as well.
How does OCT Angiography compare to dye-based angiography?
Intravenous fluorescein and indocyanine green angiography are invasive, time-consuming, and expensive methods that evaluate retinal and choroidal pathologies in a two-dimensional format, respectively. OCTA is a faster and safer imaging tool that requires no dye injection and effectively provides imaging for both retinal and choroidal circulations. In OCTA, there is no vessel filling time, no late stage hyper fluorescence, leakage, or pooling that can obscure vasculature, structures, and dimensions.1 Moreover, OCTA provides a three-dimensional high-resolution display that focuses on the fine details of the morphology and activity of superficial retinal microvasculature to the deep capillary structures within the choroid. When considering these advantages, OCTA can be thought of as an additional tool to conventional dye-based angiography in assessing vascular disease regression or progression in clinical practice today.
Figure 1: Fluorescein Angiography images show typical vessel leakage while OCTA images show precise delineation and measurement of these same vessels.
OCT Angiography Imaging Techniques
Volumetric data acquisition
Volumetric data acquisition provides sequential scans that tease out layers of the retina from the anterior superficial capillary plexus to the posterior choriocapillaris and choroid. In order to examine a specific retinal layer for pathology, this display provides the ability to highlight certain areas of vasculature and isolates blood flow information at a specific retinal depth which can be used to follow response to treatment.
Figure 2: Sequential 6x6mm OCTA maps from the anterior superficial retinal capillaries to the posterior choriocapillaris.
Retinal blood flow analysis
Blood flow analysis within the retina and choroid are created through motion contrast where several B scan images of the same cross-sectional area are taken at once and compared against each other. Once the technology accounts for head movements and saccadic eye movements, the reflectivity patterns imaged are based on red blood cell motion. This gives a superimposed blood flow overlay that is color coded to correlate data with other vasculature and structural images. Motion contrast comes in handy in deciding if vascularization of preretinal tissue and pigment epithelial detachments are present or not.