The aim of biomedical photoacoustic spectroscopy is to make quantitative, spatially resolved and non-invasive measurements of the concentration of chromophores in biological tissue. Its principle relies upon the generation of acoustic waves as a result of the absorption of short optical pulses in tissue. The absorption of the optical energy produces a rapid temperature and subsequent pressure rise resulting in ultrasonic acoustic waves that propagate to the surface of the tissue where they are detected. The amplitude of these waves is directly proportional to the local absorbed energy density, which is dependent upon the distribution of the absorption and scattering coefficients within the illuminated volume. The time history of these waves therefore provides information about the spatial distribution of the absorption coefficient and the scattering coefficient. In tissue, the spatial distribution of the absorption coefficient is determined by the local concentration of the major tissue chromophores of oxyhaemoglobin, deoxyhaemoglobin, lipids and water. The absorption of each chromophore has a characteristic wavelength dependence, which allows spectroscopic information to be obtained by making photoacoustic measurements at different excitation wavelengths. By analysing the wavelength-dependent photoacoustic response using suitable theoretical models, the local concentrations of these chromophores can be determined. This in turn allows the calculation of physiologically important parameters, such as the oxy- and deoxyhaemoglobin concentrations from which blood oxygen saturation can be calculated. Ultimately, by combining photoacoustic spectroscopy with photoacoustic imaging, the technique offers the prospect of providing high resolution quantitative three-dimensional maps of blood oxygen saturation. The technique could also be used to identify and quantify the accumulation of externally administered contrast agents such as those used in molecular imaging applications.

References

• Laufer JG, Delpy DT, Elwell CE, Beard PC, (2007) Quantitative spatially resolved measurement of tissue chromophore concentrations using photoacoustic spectroscopy: application to the measurement of blood oxygenation and haemoglobin concentration, Physics in Medicine and Biology, 52, pp141168, 2007.
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• Laufer, J, Elwell, CE, Delpy, DT, and Beard, PC (2005): In vitro measurements of absolute blood oxygen saturation using pulsed near-infrared photoacoustic spectroscopy: accuracy and resolution, Physics in Medicine and Biology 50, 4409-4428. Download PDF
• Laufer, J, Elwell,C, Delpy, D, and Beard, P (2006): Absolute measurements of local chromophore concentrations using pulsed photoacoustic spectroscopy, Proc. SPIE 6086, 60861J. Download PDF