Abstract
To date, two main categories of OCT techniques have been described for imaging hemodynamics: Doppler OCT and OCT angiography. Doppler OCT can measure axial velocity profiles and flow in arteries and veins, while OCT angiography can determine vascular morphology, tone, and presence or absence of red blood cell (RBC) perfusion. However, neither method can quantify RBC velocity in capillaries, where RBC flow is typically transverse to the probe beam and single-file. Here, we describe new methods that potentially address these limitations. Firstly, we describe a complex-valued OCT signal in terms of a static scattering component, dynamic scattering component, and noise. Secondly, we propose that the time scale of random fluctuations in the dynamic scattering component are related to red blood cell velocity. Analysis was performed along the slow axis of repeated B-scans to parallelize measurements. We correlate our purported velocity measurements against two-photon microscopy measurements of RBC velocity, and investigate changes during hypercapnia. Finally, we image the ischemic stroke penumbra during distal middle cerebral artery occlusion (dMCAO), where OCT velocimetry methods provide additional insight that is not afforded by either Doppler OCT or OCT angiography.
Highlights
Optical imaging methods [1,2,3] have significantly impacted the field of neuroimaging and are widely used to study cellular and vascular physiology in the brain
Optical imaging modalities can be broadly classified into two groups: macroscopic methods using diffuse light which achieve spatial resolutions of hundreds of μm to millimeters, and microscopic methods which achieve micron-scale resolutions
Point 1 is dominated by dynamic scattering, and point 3 is dominated by static scattering
Summary
Optical imaging methods [1,2,3] have significantly impacted the field of neuroimaging and are widely used to study cellular and vascular physiology in the brain. While macroscopic imaging methods using diffuse light can achieve high penetration depths and large fields of view, they do not provide high spatial resolution. While microscopy achieves subcellular spatial resolution, the imaging speed, penetration depth, and field of view are limited. Because the OCT depth resolution depends on the coherence length of light and not the confocal parameter (depth of field), low NA lenses may be used This contrasts with two-photon microscopy, where high depth resolution and penetration depth in scattering tissue in vivo can be achieved only with high-NA water immersion objectives. The use of relatively low NA objectives allows OCT imaging of large fields of view, enabling the synthesis of microscopic and macroscopic information. This capability is attractive for longitudinal studies where cumulative dye toxicity is a concern
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