Feature Of The Week 1/22/12: In vivo depth-resolved oxygen saturation by dual-wavelength photothermal (DWP) OCT.
All mammalian tissues need oxygen for survival. The human body has a delicately organized vascular network that supplies our tissue with oxygen and other nutrients as well as removing waist products. Not surprisingly, aberrations in vascular oxygen supply are implicated in at least 70 diseases and that number continues to grow. The oxygen distribution from vascular hemoglobin to the parenchymal cells begins with oxygen diffusion first from arterioles with diameters less than 50 µm, and second from capillaries, with deoxygenated blood draining back to venules where the blood is redirected to the lungs for re-oxygenation.
The earliest abnormalities in oxygen exchange take place at the capillary level and these can often be discerned clinically from hemoglobin oxygen saturation (SaO2) levels in arterioles and venules. Oxygen saturation (SaO2) levels in individual capillaries is frequently too diffuse and requires a great deal of processing and is not easy to interpret, while information from larger (> 50 µm diameter) arteries and veins can be dominated by nearby healthy tissue which mask localized pathology. Conversely, information acquired from target arterioles and venules ranging in diameters from 10-50 µm can provide relatively localized and early signs of oxygen delivery abnormalities that can be valuable for making clinically meaningful conclusions.
Current approaches to assess microvascular blood oxygenation include: a) invasive approaches such as oxygen-sensitive microelectrode techniques; b) minimally invasive methods with resolution coarser than 50 µm such as phosphorescence quenching imaging of exogenous oxygen sensitive dyes; c) non-invasive without depth resolution such as spectroscopic oximetry; and d) non-invasive with resolution coarser than 50 µm such as functional MRI and photoacoustic imaging.
Spectroscopic Fourier Domain Optical Coherence Tomography (SFD-OCT) has been reported to measure depth-resolved microvasculature SaO2 levels in phantoms and tissue. However, the variation and complexity of light scattering by blood and tissue limits SaO2 sensitivity of intensity-based SFD-OCT and complicates clinical translation. Recently, our group reported a Dual Wavelength Photothermal (DWP) OCT approach to measure depth-resolved microvasculature SaO2 levels in phantom blood vessels.
In DWP-OCT, incident photothermal excitation light at two distinct wavelengths is absorbed by target chromophores in a sample resulting in thermally-induced optical pathlength (op) variations that are measured with a phase-sensitive OCT system. Relative concentration of the two target chromophores can be estimated from the normalized ratio of op variations at the photothermal excitation wavelengths. Recently, we extended DWP-OCT to depth-resolved in vivo measurement of SaO2 levels in a 30 µm arteriole in a murine animal model. We believe that DWP OCT instrumentation can provide clinical and biomedical researchers with a powerful tool to improve our understanding of basic pathophysiological processes underlying the natural history of various diseases as well as biomarkers to detect the earliest stages of the disease before irreversible changes have occurred and allow monitoring of long-term progression and effectiveness of selected therapeutic interventions.
DWP OCT estimation of SaO2 levels relies on measuring optical pathlength (op) that light experiences while traveling from the probe to specific tissue layers and back to the probe. Since op is not influenced by intensity fluctuation as in intensity based spectroscopic methods we believe that further development of DWP OCT will provide an important candidate approach to image microvasculature SaO2 levels for fundamental biomedical research and clinical instrumentation.
For more information see recent Article. Courtesy Roman V. Kuranov from University of Texas Health Science Center.