Three-dimensional mapping of oxygen tension in cortical arterioles before and after occlusion
Published in Biomedical Optics Express, Vol. 4 Issue 7, pp.1061-1073 (2013)
Spotlight summary: Distributed oxygen levels in the brain have proven difficult to examine at the level of the microvasculature. Classical oximetry techniques measure bulk tissue oxygenation and typically cannot quantify microvascular levels of oxygen under baseline conditions let alone after selective flow alterations made to examine oxygenation changes. Clark electrodes have also been previously used to collect invasive measurements outside the vascular lumen of large vessels following which mathematical models of tissue oxygen delivery are constructed. However, this method is particularly invasive and perturbs the biological system during measurement collection. Recently, optical imaging techniques have been investigated to examine hemodynamic response of microvasculature beds and have demonstrated greater sensitivity and accessibility without significant physiological perturbation as compared to conventional methods. Combining laser scanning microscopy and phosphorescence quenching to quantify oxygen concentration has demonstrated microvasculature oxygen tension in tissue. However, these measurements are difficult to perform in three dimensions with conventional oxygen sensitive probes due to their low two-photon cross section and skewed calibration when tissues contain high probe concentrations. Recently, a new platinum-porphyrin phosphorescent oxygen sensor (PtP-C343) has been developed which has a larger two-photon cross section and demonstrates increased signal at low oxygen concentration.
In the work by Kazmi, et al a two-photon lifetime microscopy technique that utilizes PtP-C343 is combined with laser speckle contrast imaging (LSCI) to examine the vascular networking impact on intravascular oxygen tension. Two-photon lifetime microscopy enables deeper tissue penetration imaging due to the near-infrared wavelength of excitation light facilitating oxygen detection through PtP-C343 at increased depths over single photon lifetime microscopy. LSCI enables imaging of blood flow characterization, but is typically surface weighted. Thus combining these methods enables quantification of tissue oxygen tension at depth in live animals. Following development and optimization of the combined imaging system it was utilized to image oxygenation of microvasculature through window chambers in murine brains. Experiments were performed to quantify baseline pO2 in descending arterioles, where substantial pO2 gradients were observed with arteriole descent into the cortex. Photothrombotic clotting was used to occlude the entire lumen of an observed vessel; imaging demonstrated that intravascular pO2 immediately upstream of the clot remained consistent with baseline measurements. However, downstream there was a significant drop in pO2 compared to baseline measurements. Additional experiments were performed to examine observation of gradient reversal in the primary arteriole, which were readily quantifiable using the imaging system. This paper demonstrates the construction and validation of a combined two-photon fluorescence and phosphorescence lifetime microscope with laser speckle contrast imaging to image photothrombosis and its effect on tissue pO2. Cerebral microvasculature is used to demonstrate this technique, but similar interrogations could be performed on other highly perfused tissues with a feasible method for optical accessibility.
--Summer L. Gibbs
ToC Category: Functional Imaging
|OCIS Codes:||(110.6150) Imaging systems : Speckle imaging|
|(170.0170) Medical optics and biotechnology : Medical optics and biotechnology|
|(170.1460) Medical optics and biotechnology : Blood gas monitoring|
|(170.3650) Medical optics and biotechnology : Lifetime-based sensing|
|(170.5380) Medical optics and biotechnology : Physiology|
|(180.4315) Microscopy : Nonlinear microscopy|
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