Absorption Coefficient Measurement of Turbid Media Based on Acousto-Optical Tomography
Objective The physiological state of human tissues and the lesions of tissues are found to be closely related to the optical properties of tissues.The accurate measurement of optical parameters of tissue determines optical diagnosis correctness and phototherapy effectiveness,which is particularly essential in medical applications.At present,the common methods for measuring the optical parameters of biological tissues are integrating sphere technique,diffusion optical tomography,fluorescence imaging,and optical coherence tomography.In these methods,the measurement depth is not deep enough,or the measurement accuracy is not good enough to meet the practical applications.Acousto-optical tomography(AOT)combines the high spatial resolution of ultrasound with the high sensitivity of optical detection to provide excellent imaging depth(cm)at high imaging resolution(submm).AOT employs the localization and modulation of focused ultrasound,and the localization and quantitative measurement of the absorption coefficient of turbid media can be realized.Finally,the limitation that other measurement methods cannot consider both measurement depth and measurement accuracy can be compensated.We obtain the quantitative relationship between the absorption coefficient of the turbid medium and the acoustic-optical signal by theoretical analysis and COMSOL simulation.Furthermore,the absorption coefficient of turbid medium is measured by the AOT experiment,which preliminarily verifies the feasibility of AOT in the measurement of the tissue absorption coefficient.Methods Based on the radiation transmission theory and the intensity modulation mechanism of acousto-optic interaction in a turbid medium,the analytical relationship between acousto-optic signals and medium optical parameters is obtained.The finite element simulation software COMSOL Multiphysics is adopted for simulation,the extrapolated boundary equation and diffusion approximation theory are utilized to define the light field,and the ultrasound theory is to define the sound field.Meanwhile,the multi-physics field coupling is performed based on an intensity modulation mechanism,and an experimental system of AOT is built to measure the absorption coefficient of a turbid medium.Results and Discussions In the COMSOL simulation,the intensity of the acousto-optic signal increases linearly with the rising incident light intensity(Fig.5),and the relative intensity of the acousto-optic signal(the ratio of the acousto-optic signal intensity to the incident light intensity)decreases exponentially with the growing absorption coefficient(Fig.6).The absorption coefficient calculated by simulation is very close to the actual set value.The maximum absolute error is 0.049 cm-1,the minimum absolute error is 0.0074 cm-1,the mean absolute error is 0.026 cm-1,and the detection correlation coefficient is greater than 0.95(Fig.7).In the experiment,acousto-optic imaging is performed on samples with different absorption coefficients.When only the incident light intensity is changed,the acousto-optic signal and the incident light intensity show a linear growth relationship(Fig.10).When the other conditions remain unchanged,the relative intensity of the acousto-optic signal decreases exponentially as the absorption coefficient increases(Fig.11).The experimental results are consistent with the simulation results.The average absolute error is 0.047 cm-1 and the average relative error is 6.5%when the absorption coefficient is measured for a tissue simulation sample with a thickness of 10 mm(Fig.12).Conclusions The relationship between tissue absorption coefficient and the acousto-optic signal is analyzed theoretically by combining the radiation transmission theory of light propagating in tissue and the intensity modulation mechanism of acousto-optic interaction in turbid media.The relative value of acousto-optic signals is determined by ultrasound parameters(sound pressure,sound frequency,sound speed)and tissue parameters(thickness,optical parameters),and is independent of the incident light intensity.The relative value of the acousto-optic signal decays exponentially with the absorption coefficient of the medium when other conditions remain unchanged.The theoretical results are in good agreement with COMSOL simulations.The maximum relative error of the absorption coefficient measured in the COMSOL simulation is less than 8%.The experimental measurements are carried out using the AOT system,and the experimental results are basically consistent with the simulation results.In the experiment,the maximum absolute measurement of the absorption coefficient of the tissue simulation sample with a thickness of 10 mm is 0.082 cm-1,and the maximum relative error is 9.3%,which initially verifies the quantitative measurement feasibility of the absorption coefficient of the turbid media by AOT.AOT combines the advantages of optical and acoustic technology to measure the absorption coefficient of deep tissue.For example,by combining with a multi-wavelength light source,the absorption coefficient of blood vessels at different wavelengths can be obtained to measure their blood oxygen saturation and thus provide more references for the early diagnosis of some tumors.At present,the main problem of AOT is that the acousto-optic signals are weak with a low signal-to-noise ratio.It is a great challenge for detection instruments and detection methods to extract weak acousto-optic signals from strong background light,and it is also the key and difficult problem that AOT must solve in the future.
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