Integrating sphere-based photoacoustic setup for simultaneous absorption coefficient and Grüneisen parameter measurements with biomedical liquids

A method for simultaneously measuring the absorption coefficient μ_a and Grüneisen parameter Λ of biological absorbers in photoacoustics is designed and implemented using a coupled-integrating sphere system. A soft transparent tube with inner diameter of 0.58mm is used to mount the liquid absorbing sample horizontally through the cavity of two similar and adjacent integrating spheres. One sphere is used for measuring the sample's μ_a using a continuous halogen light source and a spectrometer fiber coupled to the input and output ports, respectively. The other sphere is used for simultaneous photoacoustic measurement of the sample's Λ using an incident pulsed light with wavelength of 750nm and a flat transducer with central frequency of 5MHz. Absolute optical energy and pressure measurements are not necessary. However, the derived equations for determining the sample's μ_a and Λ require calibration of the setup using aqueous ink dilutions. Initial measurements are done with biological samples relevant to biomedical imaging such as human whole blood, joint and cyst fluids. Absorption of joint and cyst fluids is enhanced using a contrast agent like aqueous indocyanine green dye solution. For blood sample, measured values of μ_a = 0.580 ± 0.016 mm^-1 and Λ = 0.166 ± 0.006 are within the range of values reported in literature. Measurements with the absorbing joint and cyst fluid samples give Λ values close to 0.12 which is similar to that of water and plasma.