Optical coherence tomography angiography (OCTA) has recently emerged for imaging vasculature in clinical ophthalmology. Yet, OCTA images contain artifacts that remain challenging to interpret. To help explain these artifacts, we perform contrast-enhanced OCTA with a custom-designed wide-field ophthalmoscope in rats in vivo. We choose an intravascular contrast agent (Intralipid) with particles that are more isotropically scattering and more symmetrically shaped than red blood cells (RBCs). Then, by examining how OCTA artifacts change after contrast agent injection, we attribute OCTA artifacts to RBC-specific properties. In this work, we investigate retinal and choroidal OCTA in rats with or without melanosomes, both before and after contrast agent injection, at a wavelength at which scattering dominates the image contrast (1300 nm). First, baseline images suggest that high backscattering of choroidal melanosomes accounts for the relatively dark appearance of choroidal vessel lumens in OCTA. Second, Intralipid injection tends to eliminate the hourglass pattern artifact in OCTA images of vessel lumens and highlights vertical capillaries that were previously faint in OCTA, showing that RBC orientation is important in determining OCTA signal. Third, Intralipid injection increases lumen signal without significantly affecting the tails, suggesting that projection artifacts, or tails, are due to RBC multiple scattering. Fourth, Intralipid injection increases the side-to-top signal ratio less in choroidal vessel lumens of pigmented rats, suggesting that melanosome multiple scattering makes the hourglass artifact less prominent. This study provides the first direct experimental in vivo evidence to explain light scattering-related artifacts in OCTA.
Keywords: (110.4500) Optical coherence tomography; (170.4460) Ophthalmic optics and devices; (290.4210) Multiple scattering.