Abstract
Optical coherence tomography angiography (OCTA) is a rapidly developing technology to non-invasively image retinal, choroidal, and optic nerve head blood flow. OCTA uses flow motion as intrinsic contrast, therefore interpretation differs from dye-based fluorescein and indocyanine green angiography, in which dye transit and leakage provide the primary contrast. Practical OCTA has recently become available due to the development of high-speed spectral-domain (SD) and swept-source (SS) OCT systems, and very efficient ways to compute flow signal such as the split-spectrum amplitude-decorrelation angiography algorithm.
The 3-dimensional nature of OCTA allows for detailed examination of vascular anatomy in both cross-sectional and en face displays that separate the normal vascular beds and vascular pathologies. Proper selection of slab boundaries using reference anatomic surfaces is key to visualization and recognition of pathologies.
Several special artifacts are present in OCTA. Eye motion not only causes mis-registration of scan lines (B-frames), but also produces artifactual flow signal (bright line artifacts). Superficial flow projects flickering shadow on deeper reflective structures that also presents as artifactual flow signal (projection artifacts). The projection artifact manifests as vertical tails that extend from blood vessels on cross-sectional OCTA, and reproduces superficial vascular networks on deeper layers on en face OCTA. Finally, low OCT signal due to shadowing can masquerade as nonperfusion areas. Recognizing these artifacts and suppressing them using special computer algorithm are needed for correct interpretation.
In OCTA, abnormal blood vessels can be classified according to the depth of the pathologies in neovascular age-related macular degeneration, proliferative diabetic retinopathy, central serous chorioretinopathy, and macular telangiectasia. Nonperfusion of the superficial, intermediate and deep retinal plexuses and the choriocapillaris can be visualized. Automated quantification of neovascularization and nonperfusion areas have been shown to be useful in a wide variety of retinal and choroidal vascular diseases.
In glaucoma and other optic neuropathies, the attenuation of blood flow in the disc, peripapillary retina, and macula can be visualize on en face OCT and quantified by flow index, vessel density, and nonperfusion area. These have demonstrated diagnostic value in glaucoma and multiple sclerosis. There is high correlation between OCTA parameters and visual field parameters and suggest potential for the use of OCTA in the monitoring of disease progression.
The non-invasive nature of OCT angiography will allow it to be used much more frequently than was ever possible with conventional dye-based angiography. The new generation of commercial OCT systems has sufficient speed to obtain high quality OCT angiography of the macular and disc regions. Very high-speed swept-source OCT prototypes are capable of wide-field OCTA to capture peripheral pathologies. Wide-field OCTA may be particularly useful in the evaluation of diabetic retinopathy and some inherited retinal degenerations. Clinical applications of both commercial OCTA systems and advanced prototypes will be presented.