Faculty Bibliography

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.

Most flying-spot optical coherence tomography and optical coherence microscopy (OCM) systems use a symmetric confocal geometry, where the detection path retraces the illumination path starting from and ending with the spatial mode of a single-mode optical fiber. Here we describe a visible light OCM instrument that breaks this symmetry to improve transverse resolution without sacrificing collection efficiency in scattering tissue. This was achieved by overfilling a water immersion objective on the illumination path while maintaining a conventional Gaussian mode detection path (1/e² intensity diameter ∼0.82 Airy disks), enabling ∼1.1  μm full width at half-maximum (FWHM) transverse resolution. At the same time, a ∼0.9  μm FWHM axial resolution in tissue, achieved by a broadband visible light source, enabled femtoliter volume resolution. We characterized this instrument according to paraxial coherent microscopy theory and, finally, used it to image the meningeal layers, intravascular red blood cell-free layer, and myelinated axons in the mouse neocortex in vivo through the thinned skull.

Light-scattering methods are widely used in soft matter physics and biomedical optics to probe dynamics in turbid media, such as diffusion in colloids or blood flow in biological tissue. These methods typically rely on fluctuations of coherent light intensity, and therefore cannot accommodate more than a few modes per detector. This limitation has hindered efforts to measure deep tissue blood flow with high speed, since weak diffuse light fluxes, together with low single-mode fiber throughput, result in low photon count rates. To solve this, we introduce multimode fiber (MMF) interferometry to the field of diffuse optics. In doing so, we transform a standard complementary metal-oxide-semiconductor (CMOS) camera into a sensitive detector array for weak light fluxes that probe deep in biological tissue. Specifically, we build a novel CMOS-based, multimode interferometric diffusing wave spectroscopy (iDWS) system and show that it can measure ∼20 speckles simultaneously near the shot noise limit, acting essentially as ∼20 independent photon-counting channels. We develop a matrix formalism, based on MMF mode field solutions and detector geometry, to predict both coherence and speckle number in iDWS. After validation in liquid phantoms, we demonstrate iDWS pulsatile blood flow measurements at 2.5 cm source-detector separation in the adult human brain in vivo. By achieving highly sensitive and parallel measurements of coherent light fluctuations with a CMOS camera, this work promises to enhance performance and reduce cost of diffuse optical instruments.

Optical Coherence Tomography Angiography (OCTA) refers to a powerful class of OCT scanning protocols and algorithms that selectively enhance the imaging of blood vessel lumens, based mainly on the motion and scattering of red blood cells (RBCs). Though OCTA is widely used in clinical and basic science applications for visualization of perfused blood vessels, OCTA is still primarily a