In recent years, the cases of cancer have increased making it one of the most deadly diseases with breast cancer being the most prevalent in women. An early and effective diagnosis of breast cancer is crucial to the recovery of patients. Compared with traditional optical techniques using unpolarized light, polarization imaging can bring to light the rich microstructure of biological tissues and has the advantages of easy operation, fast detection, and no need for sample staining. Polarization imaging has shown great potential for application in biological tissue analysis and has been applied to the study of pathological diagnosis of cancer. However, during cancerous tissue testing, the mechanical action such as sectioning is required for pathological section preparation. The optical properties of cancerous tissues are affected by mechanical interactions, which generate strains that alter the interrelationship of the cancerous tissue microstructure, leading to changes in the scattering microenvironment and ultimately affecting the inversion of the polarization parameters. Therefore, it is important to investigate the effect of mechanical action such as stretching on the optical properties of cancerous tissues using polarization imaging as a tool in the pathological diagnosis of cancer.

The tissue section is elongated to a certain extent in the vertical feed direction because of the compression of the blade on the tissue in the feed direction during the preparation of the tissue section. Based on this concept, this study is conducted to investigate the effect of mechanical stretching on the Mueller matrix characterization of intraductal cancerous breast tissue. As the magnitude of these mechanical actions is small, a system is developed that enables achieving mild mechanical stretches. The method uses the Mueller matrix polar decomposition method to determine two characterization parameters that can characterize the fundamental polarization properties of a medium: dichroism (D) and scattering depolarization (Δ). The mean differences of the characterization parameters between different tissue structures and their change rates under mechanical stretching are analyzed using intraductal cancerous breast tissue and normal breast tissue as the study objects.

In this study, digital staining of sections of intraductal cancerous breast tissue is performed using the D and Δ parameters. It is observed that the cancerous region within the duct and the fibrous region surrounding the duct are represented by different colors in the digitally stained images for the two parameters based on their values. As mechanical stretching progresses, there is no significant change in these parameters for the cancerous region within the ducts, while values of D for the fibrous region surrounding the ducts gradually decrease and a gradual increase in the Δ values is observed (Fig. 5). The parameter values for the two different tissue regions are then extracted, averaged and statistically plotted for determining the variation of the mean value with the stretching process and rate-of-change of the difference in the mean value between the different regions. As mechanical stretching progresses, the mean value of D in the cancerous tissue region remains essentially constant, while the mean value of D in the fibrous tissue region shows a monotonically decreasing trend. The difference between the parameters of the two tissues shows a decreasing trend, with the rate of change reaching a maximum value of 7.9% at the maximum stretching. This is probably because the fibers whose directions are different from the direction of stretching show different orientation change. Thus, the overall orientation consistency and anisotropy of fibers decrease, which leads to a decrease in the D value (Fig. 6). As mechanical stretching progresses, the mean value of Δ in the cancerous tissue region remains more or less constant, while the mean value of Δ in the fibrous tissue region shows a monotonic increase. The difference between the parameters of the two tissues shows a decreasing trend, and the rate of change reaches a maximum of 12.7% at the maximum stretching. This may be because fibers with a different orientation from the stretching direction become looser as they are mechanically stretched, which in turn leads to an increase in their overall scattering depolarization values (Fig. 7).

The precise differentiation of cancerous and normal tissues is of great importance in clinical applications. In this study, a full-polarization Mueller matrix imaging system with mechanical stretching and detection modules for biological tissues is built. The changes in the polarization representation parameters of intraductal cancerous and normal breast tissues under mechanical stretching are investigated using this system, and the differences between the same polarization representation parameters of different tissue structures under mechanical stretching and their change rates are studied. The results show that the difference between D values and the difference between Δ values for both the cancerous and normal regions tend to decrease with mechanical stretching, with a maximum rate-of-change of 7.9% for D and 12.7% for Δ. The analysis of the experimental data shows that the greater the magnitude of mechanical stretching, the greater the difference in the change trend of parameter between cancerous and normal tissues, and the closer the values of parameters between cancerous and normal tissues, i.e., the difference between cancerous and normal tissues becomes less evident. This has implications for polarimetric imaging-based probing using D and Δ parameters as the diagnostic basis, where large deformations are avoided as much as possible during mechanical handling of tissues, i.e., while biological tissue sectioning, using sharper tools and controlling tool angles. In this study, only the D and Δ parameters have been investigated. The work will be extended to focus on other polarization characterization parameters in future research.