In vivo tumor imaging of pre-clinical models via reflection-mode measurements of circular degree of polarization.

Abstract

Significance: Tumor tissues exhibit contrast with healthy tissue in circular degree of polarization (DOP) images via higher magnitude circular DOP values and increased helicity-flipping. This phenomenon may enable polarimetric tumor detection and surgical/procedural guidance applications.

Aim: Depolarization metrics have been shown to exhibit differential responses to healthy and cancer tissue, whereby tumor tissues tend to induce less depolarization; however, the understanding of this depolarization-based contrast remains limited. Therefore, we investigate depolarization signals from tumor tissue and non-tumor tissue.

Approach: Mice ( $n=3$ ) with human pancreatic ductal adenocarcinoma (PDAC) xenografts enable polarimetric comparison between tumor tissue and non-tumor tissues. Modified signed-value DOP equations aid in the interpretation of DOP images, which encode helicity-flipping and co-linearity as negative values, but still yield the same magnitudes as conventional DOP calculations.

Results: Linear DOP is greater in magnitude than circular DOP across both tissue types; however, circular DOP yields greater contrast between tumor and non-tumor tissues. Circular DOP values are higher in magnitude and more negative (i.e., more helicity-flipping) in tumors, whereas linear DOP values exhibit similar behavior; however, they are only slightly higher in magnitude and slightly more negative (i.e., more co-linearity) in tumors.

Conclusions: Circular DOP images yield useful contrast between human PDAC xenografts and surrounding healthy skin in live mice. Each tumor region exhibited higher magnitude circular DOP (and total DOP) values, as previously observed. We noted three indications of Rayleigh scattering in the tumor tissue: (1) linear DOP > circular DOP, (2) helicity-flipping > helicity-preservation, and (3) co-linear intensity > cross-linear intensity. Rayleigh scatterers have been found to be highly polarization preserving; thus, we posit that higher DOP in tumor tissues may arise from an increased presence of Rayleigh scatterers. Furthermore, circular DOP may yield greater contrast between tumor and non-tumor via its well-observed sensitivity to scatterer size. Further investigation is warranted to test these hypotheses.