Optical assessment of tissue anisotropy in <italic>ex vivo</italic> distended rat bladders

Abstract

Abstract. Microstructural remodelling in epithelial layers of various hollow organs, including changes in tissue anisotropy, are known to occur under mechanical distension and during disease processes. In this paper, we analyze how bladder distension alters wall anisotropy using polarized light imaging (followed by Mueller matrix decomposition). Optical retardance values of different regions of normal rat bladders under different distension pressures are derived. Then optical coherence tomography is used to measure local bladder wall thicknesses, enabling the calculation of the tissue birefringence maps as a measure of the tissue anisotropy. Selected two-photon microscopy is also performed to better understand the compositional origins of the obtained anisotropy results. The dome region of the bladder shows maximum birefringence when the bladder is distended to high pressures, whereas the ventral remains roughly isotropic during distension. In addition, the average anisotropy direction is longitudinal, along the urethra to dome. The derived wall anisotropy trends are based on birefringence as an intrinsic property of the tissue organization independent of its thickness, to aid in understanding the structure-functions relation in healthy bladders. These new insights into the wall microstructure of ex vivo distending bladders may help improve the functionality of the artificially engineered bladder tissues.

Flexible polarimetric probe for 3 × 3 Mueller matrix measurements of biological tissue

“…Mechanical stretching induces increased tissue anisotropy (an increase in optical birefringence), for example as shown by Alali et al . in ex vivo distended rat bladders 22 . The tissue was fixed in 4% paraformaldehyde for 24 hours, changing its polarisation properties slightly 23 , but enabling much easier tissue handling, stretching, and measurement.…”

Detecting axial heterogeneity of birefringence in layered turbid media using polarized light imaging

“…One of these microstructural characteristics is anisotropy caused by fiber (and/or cellular) alignment which can be quantified as birefringence [5]. Birefringence is usually derived from the tissue’s Mueller matrix and polarimetric measurements [3,4,6–10]. But tissue properties, including birefringence, may be spatially varying; in particular, layered tissue often exhibits depth-dependent properties in its different layers.…”

“…The depth averaged Mueller matrix can be experimentally measured in biological tissues both in transmission and backscattering mode [17]. Depending on the tissue’s optical properties (absorption, scattering magnitude and anisotropy) and the detection angle, the Mueller matrix can be representative of few millimeters deep into the tissue [3,17,18]. Of great importance is the cumulative phase retardance δ ( = 2π·Δn·d/λ) which is proportional to the birefringence Δn and the average pathlength of photons d at wavelength λ. Retardance can be found from polar decomposition of Mueller matrix.…”

“…The matrix M R can be further decomposed to a linear part and a circular part [20,21]. From the linear part M LR , the retardance orientation (slow axis) can be obtained as [2,3]:…”

“…Another parameter derived from polar decomposition is the retardance ellipticity which may give an indication of birefringence (in)homogeneity [3,20,21]. However, without resorting to polar decomposition analysis and assumptions, we note that the Mueller matrix itself possesses interesting symmetry properties, depending on the sample’s composition and structure, and thus may shed light on the tissue birefringence (in)homogeneity problem.…”