Contrasting transmission and backreflection configurations in polarimetric imaging

Mueller matrix polarimetry is a powerful tool for biomedical imaging, but its two main modalities, transmission and backreflection, are often incorrectly considered equivalent. Transmission is typically used in ex vivo applications such as histology, where light passes through thin tissue layers with minimal scattering, enabling clear interpretation of properties like linear birefringence. In contrast, backreflection, commonly used in in vivo imaging, involves complex light interactions including multiple scattering and mixed diffuse and specular reflection. Specular highlights can distort the measured Mueller matrix and lead to inaccurate tissue characterization due to dynamic range limitations. In this work, we demonstrate that transmission-based models are not valid for backreflection by comparing skeletal muscle, brain, and skin tissues. We observe distinct polarimetric regimes: while retardance is the dominant feature in transmission, it is washed out in bulk muscle backreflection due to phase scrambling, leaving diattenuation as the surviving observable. Conversely, brain white matter retains a measurable retardance signal due to the specific scattering nature of myelin. Finally, in skin, we show that intrinsic anisotropy is largely masked by epidermal scattering and surface artifacts, though scalar metrics such as the Degree of Polarization can still reveal structural changes associated with aging.