Abstract
We demonstrate in vivo measurements in human retinal vessels of an experimental parameter, the slope of the low coherence interferometry (LCI) depth reflectivity profile, which strongly correlates with the real value of blood hematocrit. A novel instrument that combines two technologies, spectral domain low coherence interferometry (SDLCI) and retinal tracking, has been developed and used for these measurements. Retinal tracking allows a light beam to be stabilized on retinal vessels, while SDLCI is used for obtaining depth-reflectivity profiles within the investigated vessel. SDLCI backscatter extinction rates are obtained from the initial slope of the A-scan profile within the vessel lumen. The differences in the slopes of the depth reflectivity profiles for different subjects are interpreted as the difference in the scattering coefficient, which is correlated with the number density of red blood cells (RBC) in blood. With proper calibration, it is possible to determine hematocrit in retinal vessels. Ex vivo measurements at various RBC concentrations were performed to calibrate the instrument. Preliminary measurements on several healthy volunteers show estimated hematocrit values within the normal clinical range.
Highlights
Blood hematocrit is defined as the volume percentage of the blood that consists of red blood cells (RBC’s)
We demonstrate in vivo measurements in human retinal vessels of an experimental parameter, the slope of the low coherence interferometry (LCI) depth reflectivity profile, which strongly correlates with the real value of blood hematocrit
This developing instrumentation platform can be augmented for many other physiological monitoring tasks, including blood flow, pulse profile, other in vivo flow cytometry applications, stimulus/response metrics of cognitive function, and others
Summary
Blood hematocrit is defined as the volume percentage of the blood that consists of red blood cells (RBC’s). On a periodic or continuous monitoring is essential for patients with blood disorders, in emergency situations, post operatively, or with therapies that compromise blood-forming functions and organs. Several noninvasive methods have been proposed for hematocrit measurements. Ultrasound-based continuous hematocrit measurement has been proposed by Johner et al [1]. They determined the value of hematocrit by monitoring changes in ultrasound wave velocity propagation in plasma as a function of RBC’s concentration. They reported good correlation between their measurements and the real value of the hematocrit, they noted that the uncertainty of the method depended markedly on even minute temperature variations.
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