Conghui Shao

As a vectorial property, polarization encodes high-dimensional information of light.^{1 – 3} Polarimetry can provide abundant optical and structural information of tissue; thus it has shown great potential in biomedical and clinical applications.^{4 – 10} After interacting with tissue, the polarization state of light can be altered by different structures.¹¹ By measuring the polarization state changes of output light from tissue, several properties, such as birefringence, dichroism, and depolarization, can be quantified.¹² For microscopic polarization imaging of thin tissue slices, linear birefringence is a valuable and prevalently used metric for anisotropic structures characterization.^{13 – 15} For instance, Wood et al.¹⁶ demonstrated that linear birefringence can be adopted for in vivo tissue fiber characterization. Furthermore, Pierangelo et al.¹⁷ showed that the reducing of linear birefringence resulting from the breaking down of well-ordered fibrous structures can be used for cervical cancer identification.

Polarimetry is attracting increasing attention nowadays, due to its unique ability to obtain microstructural and optical information of tissue samples label-free.^{1 – 4} Among the available polarimetric techniques, Mueller matrix (MM) imaging can comprehensively characterize the polarization-related properties of media, including diattenuation, retardance, and depolarization, which are of great importance in biomedical studies and clinical trials.^{5 – 7} Recently, transmission MM microscopy has shown potential in detection of various abnormal and pathological tissues, such as breast cancer,⁸ cervical cancer,⁹ colon cancer,¹⁰ skin cancer,^11,12 liver cirrhosis,^13,14 gastrointestinal cancer,^15,16 thyroid cancer,¹⁷ and lung cancer.¹⁸ Meanwhile, the backscattering MM polarimetry provides a promising non-contact and quantitative tool for ex-vivo measurement of bulk tissue samples,^{6,7,19 – 22} in-vivo imaging of skin tissue,^{23 – 26} and endoscopic detection of internal organs.^{27 – 31}

Polarization imaging can quantitatively probe the microscopic structure of biological tissues which can be complex and consist of layered structures. In this paper, we established a fast-backscattering Mueller matrix imaging system to characterize the dynamic variation in the microstructure of single-layer and double-layer tissues as glycerin solution penetrated into the samples. The characteristic response of Mueller matrix elements, as well as polarization parameters with clearer physics meanings, show that polarization imaging can capture the dynamic variation in the layered microstructure. The experimental results are confirmed by Monte Carlo simulations. Further examination on the accuracy of Mueller matrix measurements also shows that much faster speed has to be considered when backscattering Mueller matrix imaging is applied to living samples.