Quantitative photoacoustic measurement of blood oxygen saturation in vivo aided by an optical contrast agent

The characteristic absorption spectrum of hemoglobin depends upon the amount of oxygen the hemoglobin carries. This property of the hemoglobin enables one to image blood oxygen saturation distribution in biological tissues by using spectroscopic photoacoustic tomography. In photoacoustic imaging, the amplitude of photoacoustic signal induced by optical absorption is proportional to light energy deposition which is the product of the optical absorption coefficient and local light fluence at the imaging target. Since the attenuation of light in biological tissues are wavelength dependent, the spectrum of local light fluence at a target tissue beneath the sample surface is different from the spectrum of the incident light fluence above the surface. An unknown spectrum of the light fluence in the sample prevents us from obtaining quantitative functional images such as oxygen saturation and hemoglobin concentration in the sample by photoacoustic tomography. We developed a new technique of using an optical contrast agent with known optical absorption spectrum to obtain the accurate spectrum of light fluence at a subsurface target tissue such as a blood vessel beneath the sample surface. The technique has been validated by obtaining an accurate absorption spectrum of a micro-flow vessel buried in strong optical scattering media including diluted whole milk and chicken breast tissue. In this work, we further explored the capability of this technique through the experiments on tissue mimicking phantoms and living animals. By using this technique we were able to obtain accurate blood oxygen saturation in vessels buried at different depths in an optical scattering medium. Also, the oxygenation levels in main arteries in rat tails have been quantified more accurately in a noninvasive manner.