Noninvasive label-free imaging of microhemodynamics by optical-resolution photoacoustic microscopy

Abstract

In vivo microcirculatory imaging facilitates the fundamental understanding of many major diseases. However, existing techniques generally require invasive procedures or exogenous contrast agents, which perturb the intrinsic physiology of the microcirculation. Here, we report on optical-resolution photoacoustic microscopy (OR-PAM) for noninvasive label-free microcirculatory imaging at cellular levels. For the first time, OR-PAM demonstrates quantification of hemoglobin concentration and oxygenation in single microvessels down to capillaries. Using this technique, we imaged several important yet elusive microhemodynamic activities-including vasomotion and vasodilation-in small animals in vivo. OR-PAM enables functional volumetric imaging of the intact microcirculation, thereby providing greatly improved accuracy and versatility for broad biological and clinical applications.

Fig. 1

A representative microvascular network in a nude mouse ear imaged in vivo by OR-PAM. (a) MAP image. (b) 3D morphology (Media 1).

Fig. 2

Structural and functional microvascular imaging by OR-PAM in a nude mouse ear in vivo. (a) Structural image acquired at 570 nm. (b) Vessel-by-vessel sO₂ mapping based on dual-wavelength (570 nm and 578 nm) measurements. The calculated sO₂ values are shown in the color bar. PA: photoacoustic signal amplitude. A1: a representative arteriole; V1: a representative venule. Yellow dashed line: the B-scan position for Fig. 3.

Fig. 3

Vasomotion and vasodilation in response to switching the physiological state between systemic hyperoxia and hypoxia. (a) B-scan monitoring of the changes in the cross section of arteriole A1 (Media 2). (b) B-scan monitoring of the changes in the cross section of venule V1. (c) Changes in arteriolar and venous diameters in response to changes in physiological state (raw data were smoothed via 60-point moving averaging to isolate the effect of vasodilation). (d) Power spectrum of the arteriolar vasomotion tone. (e) Power spectrum of the venous vasomotion tone.