Abstract
Microvascular pressure drives perfusion in tissues but is difficult to measure. A method is proposed here to estimate relative pressures in microvessels using photoacoustic and ultrasound tracking of small vessels during calibrated tissue compression. A photoacoustic–ultrasound dual imaging transducer is used to directly compress on tissue in vivo. Photoacoustic signals from blood vessels diminish as an external load is applied and eventually reaches a minimum or vanishes when external pressure is sufficiently greater than the internal pressure. Two methods were proposed to estimate relative pressures. In the first approach, vessels were tracked during compression and when the vessel photoacoustic signals vanished below a set threshold, the internal pressures were assigned as the external loading pressure at the respective collapse point. In this approach pressures required to collapse vessel signatures completely were found to be much greater than physiological blood pressures. An alternative approach was to track the cross-sectional area of small vessels with changing external load and fitting the data to a known Shapiro model for thin-walled vessel compression. This approach produced estimates of internal pressures which were much more realistic. Both approaches produced the same rank-ordering of relative pressures of various vessels in vivo. Approaches thus far require future work to become fully quantitative but the present contributions represent steps towards this goal.
Highlights
Microcirculation is a clinically important factor in determining many disease conditions including septic-shock, diabetes, hypertension, ischemia, cancer, and organ failure [1,2,3,4]
Pressure in vascular networks is a key parameter to driving perfusion and drug delivery efficacy
Compression ultrasound has been used to estimate blood pressures in brachial arteries [13]. None of these approaches are suitable for estimating blood pressures in small vessels
Summary
Microcirculation is a clinically important factor in determining many disease conditions including septic-shock, diabetes, hypertension, ischemia, cancer, and organ failure [1,2,3,4] Various imaging techniques, both invasive and non-invasive, such as orthogonal polarization spectral imaging (OPS), phase contrast magnetic resonance imaging (MRI), intravital microscopy (IVM), capillary microscopy, optical coherence tomography (OCT), ultrasound biomicroscopy, and photoacoustic microscopy have become important tools in understanding blood flow in small vessels [4,5,6,7]. Both invasive and non-invasive, such as orthogonal polarization spectral imaging (OPS), phase contrast magnetic resonance imaging (MRI), intravital microscopy (IVM), capillary microscopy, optical coherence tomography (OCT), ultrasound biomicroscopy, and photoacoustic microscopy have become important tools in understanding blood flow in small vessels [4,5,6,7] These approaches, provide little information about pressures in microvessels as downstream vascular resistance is typically unknown. None of these approaches are suitable for estimating blood pressures in small vessels
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