Several experimental techniques have been developed to measure retinal blood flow.
24,25 Among these, laser Doppler velocimetry and Doppler optical coherence tomography (OCT) provide label-free measurements of blood velocity but suffer from low spatial resolution, and, hence, they are not suitable for capillary vessels. Quantifying blood flow in capillaries is important to understanding disease progression because these vessels are most vulnerable to hemodynamic and morphological alteration.
19–22,26–29 OCT angiography (OCTA) has been used to study capillary perfusion.
30–32 However, it does not provide quantitative measures of blood velocity. In recent years, adaptive optics methods have been developed that provide quantitative measures of single blood cell velocity and can achieve higher spatial resolution necessary for capillary vessels.
33,34 These approaches have been used to provide spatial and temporal variation of blood flow in single, targeted vessels and in multiple interconnected capillary vessels.
35–41 It has also been used to visualize blood cells in capillaries, and to measure cross-sectional blood velocity profiles.
36,39,41 Apart from these noninvasive techniques, confocal imaging with fluorescently labeled erythrocytes has also been used to quantify retinal blood flow in animal models.
42 One limitation of the imaging techniques, which rely on cell tracking, is that the measured cell velocity may not represent actual blood velocity.
43–46 Furthermore, because RBC distribution in the microvasculature is spatially and temporarily heterogeneous, the measurement accuracy may vary from vessel to vessel.
47,48 Also, measurement of the cross-sectional velocity and RBC distribution profiles remains difficult and not reported, barring reference 39, which gave two-dimensional (2D) velocity profiles in vessels
\( \mathbin{\lower.3ex\hbox{$\buildrel>\over {\smash{\scriptstyle \sim}\vphantom{_x}}$}} \) 20 µm diameter.