The Muller matrix, as a method for characterizing the polarization properties of samples, contains complete information about the polarization properties of samples, and it has become an important indicator for characterizing pathological tissues in basic and preclinical studies. However, in the traditional polarized light imaging method for measuring the Muller matrix, the scattering depth of polarized light in collagen tissue cannot be controlled. The obtained Muller matrix information is the average of unknown depths in collagen tissue, and it is impossible to accurately measure the Muller matrix information of the pathological tissue area. Polarized spatial frequency domain imaging (PSFDI), which combines spatial frequency domain imaging (SFDI) and polarized light imaging, is applied to measure the optical properties of biological tissues accurately.

SFDI relates the spatial frequency of the projected stripe pattern to the penetration depth of the detected light, and the imaging depth can be controlled by controlling the spatial frequency of the projected light. We designed and validated a polarization SFDI system that uses the SFDI technique to control the imaging depth, projects the streak pattern onto the surface of the measured tissue, constructs a polarizer and detector to modulate the polarization state of the polarized light, and then acquires the image data using a CMOS camera and calculates the Mueller matrix information.

Experimental results showed that the grey-scale plate diffuse reflectance measured by the polarization SFDI system was linearly correlated with the standard value (R²=0.99988). The depolarization coefficient tends to be proportional to the fat emulsion volume fraction, the two-way attenuation coefficient increases with the increase of two-way attenuation owing to the two-way attenuator, and the accurate measurement of the phase delay of the quarter-wave and full-wave plates indicates that the system can accurately measure the sample polarization parameters. A comparison of uniform light field illumination and polarization-sensitive SFDI shows that the latter effectively controls the depth and accurately measures the shallow Mueller matrix of the sample. The results of this study are expected to effectively improve the accuracy of the detection of polarization characteristics of superficial tissues and promote early tumor detection.

In this study, a PSFDI system is developed based on polarized light imaging and SFDI, and the device structure, measurement method, and data processing method are introduced. By performing the error calibration of the PSFDI device, the measurement error of the device can be less than 2%. By performing Mueller matrix imaging on tissues, we verified the reliability of the device to measure tissue polarization Mueller and the accuracy of the Mueller matrix decomposition; hence, PSFDI can be used to obtain the optical properties of various samples. The PSFDI can accurately image pathological regions, providing accurate physiological parameters for pathological analysis, and it has a wide range of biomedical applications.