The optical parameters of biological tissues can reflect their physiological state to a certain extent and provide an important reference basis for clinical diagnosis. Therefore, it is of great significance to measure the optical parameters of biological tissues. The commonly used methods for measuring the optical parameters of biological tissues have problems. Diffusion optical tomography has a deep imaging depth, but it relies on the depth learning algorithm of the simulated dataset, and its accuracy in practical applications is debatable. Optical coherence tomography, which has a high measurement accuracy, is only applicable to the measurement of optical parameters of shallow tissues. The direct measurement of scattering coefficients using a transmission model leads to a large error, and it cannot meet the requirements for measurement accuracy. Acousto-optic tomography (AOT) effectively combines the advantages of optical and acoustic technologies, and is expected to realize high-precision quantitative measurement of scattering coefficients of thick tissues. In this study, the feasibility of using acousto-optic signals to measure the scattering coefficients of tissues is confirmed by theory, finite element simulation, and experiment, and the advantages and disadvantages of the two types of measurement methods based on acousto-optic signals are compared.

Combining the diffuse theory of light propagation in biological tissues with the intensity modulation mechanism of acousto-optic interaction, the relationship between acousto-optic signals and the scattering coefficient is obtained. The finite element software COMSOL Multiphysics is used to simulate the acousto-optic process in the tissue to verify the correctness of the theoretical analysis results. In the AOT experiment, the peak-to-peak value and relative intensity of the acousto-optic signals are obtained by fixing the incident intensity and changing the incident intensity, respectively. Combining the relationship between acousto-optic signals and the scattering coefficient, the quantitative measurement of the scattering coefficient of the simulated tissue fluid is realized.

In the COMSOL Multiphysics simulation and AOT experiment, the peak-to-peak value of the acousto-optic signal shows a linear increasing relationship with the incident intensity (Fig. 5 and Fig. 10), and reveals an exponential decay trend with the scattering coefficient [Fig. 6(b) and Fig. 11(b)]. The relative intensity of the acousto-optic signal does not change with the change of the incident intensity (Fig. 5 and Fig. 10), and shows the same exponential decay relationship with the scattering coefficient [Fig. 6(a) and Fig. 11(a)]. The scattering coefficient of the medium is measured by the peak-to-peak value and relative intensity of the acousto-optic signal obtained by the simulation. The relative errors of the scattering coefficients obtained by both methods are within 0.5% (Fig. 7). The measurement accuracy of the former method is slightly better than that of the latter in the COMSOL Multiphysics simulation. In the AOT experiments, the maximum absolute error obtained using the relative intensity measurement method is 0.26 cm^-1, the average absolute error is 0.10 cm^-1, the maximum relative error is 3.88%, and the average relative error is 1.32% [Fig. 12(a)]. The maximum absolute error obtained using the peak-to-peak measurement method is 0.31 cm^-1, the average absolute error is 0.12 cm^-1, the maximum relative error is 3.34%, and the average relative error is 1.35% [Fig. 12(b)]. Under the same conditions, the measurement range of medium scattering coefficients using the relative intensities of acousto-optic signals is larger than that using the peak-to-peak values of acousto-optic signals [Fig. 13(a)].

In this study, the quantitative relationships between the peak-to-peak value and relative intensity of acousto-optic signals and the scattering coefficient of tissues are obtained. The peak-to-peak values of the acousto-optic signals show a linear incremental relationship with the incident intensity, but the relative intensity remains unchanged with the change in incident intensity. The relative intensity and peak-to-peak values of the acousto-optic signals show the same exponential decay trend with the increment of the scattering coefficient. The theoretical conclusions are verified through a COMSOL Multiphysics simulation and experiment. In the COMSOL Multiphysics simulation, the relative errors of the scattering coefficients based on the peak-to-peak values and relative intensities of the acousto-optic signals are both within 0.5%. In the AOT experiment, the maximum relative error of the scattering coefficient measured using the relative intensity of the acousto-optic signal is 3.88%, and the average relative error is 1.32%. The maximum relative error of the scattering coefficient measured using the peak-to-peak value of the acousto-optic signal is 3.34%, and the average relative error is 1.35%. It can be observed that the measurement accuracies of the two methods are comparable. In practice, the peak-to-peak value measurement method is fast, but the relative intensity measurement method can measure a larger range of the scattering coefficient. The above conclusions initially indicate the feasibility of high-precision quantitative measurement of scattering coefficients of biological tissues using acousto-optic signals. This is expected to provide a novel and non-invasive technical means for detecting biochemical attributes such as blood glucose, triglyceride, and total cholesterol concentrations in human blood tissues and can provide a certain reference for the clinical diagnosis of related diseases.