We present a quantitative study of depolarization in biological tissues and correlate it with measured optical properties (reduced scattering and absorption coefficients). Polarized light imaging was used to examine optically thick samples of both isotropic (liver, kidney cortex, and brain) and anisotropic (cardiac muscle, loin muscle, and tendon) pig tissues in transmission and reflection geometries. Depolarization (total, linear, and circular), as derived from polar decomposition of the measured tissue Mueller matrix, was shown to be related to the measured optical properties. We observed that depolarization increases with the transport albedo for isotropic and anisotropic tissues, independent of measurement geometry. For anisotropic tissues, depolarization was higher compared to isotropic tissues of similar transport albedo, indicating birefringence-caused depolarization effects. For tissues with large transport albedos (greater than ∼ 0.97), backscattering geometry was preferred over transmission due to its greater retention of light polarization; this was not the case for tissues with lower transport albedo. Preferential preservation of linearly polarized light over circularly polarized light was seen in all tissue types and all measurement geometries, implying the dominance of Rayleigh-like scattering. The tabulated polarization properties of different tissue types and their links to bulk optical properties should prove useful in future polarimetric tissue characterization and imaging studies.
Blood tests are an essential tool in clinical medicine with the ability diagnosis or monitor various diseases and
conditions; however, the complexities of these measurements currently restrict them to a laboratory setting. P&P Optica
has developed and currently produces patented high performance spectrometers and is developing a spectrometer-based
system for rapid reagent-free blood analysis. An important aspect of this analysis is the need to extract the analyte
specific information from the measured signal such that the analyte concentrations can be determined. To this end,
advanced chemometric methods are currently being investigated and have been tested using simulated spectra. A blood
plasma model was used to generate Raman, near infrared, and optical rotatory dispersion spectra with glucose as the
target analyte. The potential of combined chemometric techniques, where multiple spectroscopy modalities are used in a
single regression model to improve the prediction ability was investigated using unfold partial least squares and
multiblock partial least squares. Results show improvement in the predictions of glucose levels using the combined
methods and demonstrate potential for multiblock chemometrics in spectroscopic blood analysis.
Myocardial infarction leads to structural remodeling of the myocardium, in particular to the loss of cardiomyocytes due to necrosis and an increase in collagen with scar formation. Stem cell regenerative treatments have been shown to alter this remodeling process, resulting in improved cardiac function. As healthy myocardial tissue is highly fibrous and anisotropic, it exhibits optical linear birefringence due to the different refractive indices parallel and perpendicular to the fibers. Accordingly, changes in myocardial structure associated with infarction and treatment-induced remodeling will alter the anisotropy exhibited by the tissue. Polarization-based linear birefringence is measured on the myocardium of adult rat hearts after myocardial infarction and compared with hearts that had received mesenchymal stem cell treatment. Both point measurement and imaging data show a decrease in birefringence in the region of infarction, with a partial rebound back toward the healthy values following regenerative treatment with stem cells. These results demonstrate the ability of optical polarimetry to characterize the micro-organizational state of the myocardium via its measured anisotropy, and the potential of this approach for monitoring regenerative treatments of myocardial infarction.
We have developed a novel turbid polarimetry platform for characterization of biological tissues. Currently, we are
exploring the use of this platform for characterization of the extracellular matrix particularly for use in monitoring
regenerative treatments of myocardial infarctions. Collagen is a fibrous protein and exhibits birefringence due to
different refractive indices parallel and perpendicular to the direction of the fibers. As a result, changes in the collagen
content and organization in the tissue lead to changes in birefringence. We demonstrate our ability to measure these
extracellular changes in vivo using a mouse dorsal window chamber model. Collagenase was injected into a region of the
chamber to denature the extracellular matrix. Birefringence measurements show a large decrease in birefringence
associated with the destruction of collagen fibers. Birefringence measurements were also made through ex vivo
myocardial tissues from rats with induced myocardial infarctions including a number that had undergone regenerative
treatment with mesenchymal stem cells. Results show a decrease in birefringence from normal to infracted myocardium,
indicating a decrease in tissue organization associated with scar formation, however, an increase in birefringence was
seen in those myocardial tissues that had undergone regenerative treatment indicating reorganization of tissue structure.
Extraction / unique interpretation of the intrinsic polarization parameters in optically thick turbid media such as tissues is
complex due to multiple scattering effects and due to simultaneous occurrences of many polarization effects (the most
common polarimetry effects in tissues are depolarization, linear birefringence and optical activity). Each of these
polarimetry characteristics, if separately extracted, holds promise as a useful biological metric. We have recently
investigated the use of an expanded Mueller matrix decomposition method to tackle this problem, with early indications
showing promise. However, for further insight and for practical realization of this approach, it is essential to have
quantitative understanding of the confounding effects of scattering, the propagation path of multiply scattered photons
and detection geometry on the Mueller matrix-derived polarization parameters (parameters of particular biomedical
importance are linear retardance, optical rotation and depolarization). The effect of the ordering of the individual
matrices in the decomposition analysis on the derived polarization parameters also needs to be studied. We have
therefore investigated these issues by decomposing the Mueller matrices generated with a polarization sensitive Monte
Carlo model, capable of simulating all the simultaneous optical (scattering and polarization) effects. The results show
that with appropriate choice of detection position, indeed the inverse decomposition analysis enables one to decouple and
quantify the individual intrinsic polarimetry characteristics despite their simultaneous occurrence, even in the presence of
the numerous complexities due to multiple scattering. The details of these results are presented and the implications of
these in diagnostic photomedicine are discussed.
Myocardial infarction leads to remodeling of the myocardium, resulting in a deterioration of cardiac function. This
remodeling involves changes in the extracellular matrix, particularly an increase in collagen. Recently developed stem
cell based regenerative treatments have been shown to reduce myocardial remodeling and collagen formation after
infarction leading to an improvement in overall cardiac function. However, this emerging field is in dire need of
biomarkers to monitor the progress and success of these treatments. Collagen is a fibrous protein and exhibits
birefringence due to different refractive indices parallel and perpendicular to the direction of the fibers. As a result,
changes in the collagen content and organization in the myocardium should lead to changes in birefringence.
Birefringence measurements were made through ex vivo myocardial tissues from rats with induced myocardial
infarctions including a number that had undergone regenerative treatment with mesenchymal stem cells. Results show a
decrease in birefringence from normal to infracted myocardium, indicating a decrease in tissue organization associated
with scar formation, however, an increase in birefringence was seen in those myocardial tissues that had undergone
regenerative treatment indicating reorganization of tissue structure. These results demonstrate promise for this technique
and are motivating further work towards performing measurements in vivo.
We demonstrate the first in vivo use of a Mueller matrix decomposition method for polarization-based characterization of tissue. Collagenase is injected into a region of dermal tissue in a dorsal skin window chamber in a nude mouse to alter the structure of the extracellular matrix. Mueller matrices for polarized light transmitted through the window chamber in the collagenase-treated region, as well as a distal control region, are measured. From the measured matrices, the individual constituent polarization properties of the tissue are extracted through polar matrix decomposition. Large decreases in birefringence and depolarization are seen in the collagenase-treated region due to the destruction of collagen, showing the potential for this method to monitor the organization and structural anisotropy of tissue. This study represents the first in vivo demonstration of a Mueller matrix decomposition method for polarimetric tissue characterization.
The use of a combined spectral intensity and polarization signals optically scattered by tissue to determine analyte concentration in optically clear and turbid biological media was explored in a simulation study. Blood plasma was chosen as the biological model and glucose as the analyte of interest. The absorption spectrum and optical rotatory dispersion were modeled using experimental data and the Drude's equation, respectively, between 500 and 2000 nm. A polarization-sensitive Monte Carlo light-propagation model was used to simulate scattering media. Unfold partial least squares and multiblock partial least squares were used as regression methods to combine the spectral intensity and polarization signals, and to predict glucose concentrations in both clear and scattering models. The results show that the combined approaches produce better predictive results in both clear and scattering media than conventional partial least squares analysis, which uses intensity or polarization spectra independently. This improvement was somewhat diminished with the addition of scattering to the model, since the polarization signals were reduced due to multiple scattering. These findings demonstrate promise for the combined approach in clear or moderately scattering biological media; however, the method's applicability to highly scattering tissues is yet to be determined. The methodology also requires experimental validation.
Linear birefringence and optical activity are two common optical polarization effects present in biological tissue, and determination of these properties has useful biomedical applications. However, measurement and unique interpretation of these parameters in tissue is hindered by strong multiple scattering effects and by the fact that these and other polarization effects are often present simultaneously. We have investigated the efficacy of a Mueller matrix decomposition methodology to extract the individual intrinsic polarimetry characteristics (linear retardance and optical rotation , in particular) from a multiply scattering medium exhibiting simultaneous linear birefringence and optical activity. In the experimental studies, a photoelastic modulation polarimeter was used to record Mueller matrices from polyacrylamide phantoms having strain-induced birefringence, sucrose-induced optical activity, and polystyrene microspheres–induced scattering. Decomposition of the Mueller matrices recorded in the forward detection geometry from these phantoms with controlled polarization properties yielded reasonable estimates for and parameters. The confounding effects of scattering, the propagation path of multiple scattered photons, and detection geometry on the estimated values for and were further investigated using polarization-sensitive Monte Carlo simulations. The results show that in the forward detection geometry, the effects of scattering induced linear retardance and diattenuation are weak, and the decomposition of the Mueller matrix can retrieve the intrinsic values for and with reasonable accuracy. The ability of this approach to extract the individual intrinsic polarimetry characteristics should prove valuable in diagnostic photomedicine, for example, in quantifying the small optical rotations due to the presence of glucose in tissue and for monitoring changes in tissue birefringence as a signature of tissue abnormality.
Recently, the use of polarized light for medical diagnosis and therapeutic management has seen increased interest due
the noninvasive nature of light-tissue interactions. Examples of the use of polarized light include polarization imaging to
enhance spatial resolution in turbid media, selective imaging of polarized light to increase surface contrast in tissue,
polarization-sensitive optical coherence tomography (PS-OCT), and glucose monitoring. With these emerging
applications there is a need for controllable phantoms to validate the emerging techniques; however, this has been done
only to a limited degree primarily due to the difficulty in creating controllable phantoms. The primary effects of tissue
on the polarization of light are scattering, linear birefringence, and optical activity (circular birefringence). An ideal
phantom would exhibit all these effects simultaneously in a controllable fashion. We have achieved this through the use
of polyacrylamide gels with polystyrene microspheres added as scattering particles, strain applied to the gels to create
birefringence, and sucrose added for optical activity. The phantom methodology has been validated using our
polarimetry system. Currently, the phantom system is being used to extend our work in birefringence mapping of the
myocardium and to further our work in characterizing tissue.
A Monte Carlo model for polarized light propagation in birefringent, optically active, multiply scattering media is developed in an effort to accurately represent the propagation of polarized light in biological tissue. The model employs the Jones N-matrix formalism to combine both linear birefringence and optical activity into a single effect that can be applied to photons as they propagate between scattering events. Polyacrylamide phantoms with strain-induced birefringence, sucrose-induced optical activity, and polystyrene microspheres as scattering particles are used for experimental validation. Measurements are made using a Stokes polarimeter that detects scattered light in different geometries, and compared to the results of Monte Carlo simulations run with similar parameters. The results show close agreement between the experimental measurements and Monte Carlo calculations for phantoms exhibiting turbidity and birefringence, as well as for phantoms exhibiting turbidity, birefringence, and optical activity. Other scattering-independent polarization properties can be incorporated into the developed Jones N-matrix formalism, enabling quantification of the polarization effects via an accurate polarization-sensitive Monte Carlo model.
The effects of turbid chiral media on light polarization are studied in different directions around the scattering samples using a refined linear Stokes polarimeter, which simplifies the signal analysis, and allows for the detailed investigations of scattered light. Because no moving parts are involved in a measurement at a specific detection direction, the determination accuracy of polarization states is increased. The results show that light depolarization increases with both turbidity and detection angle for low and moderately turbid samples; however, the angular dependence decreases with increasing turbidity. When the turbidity is increased to ~100 cm–1, the depolarization becomes higher in the forward than in the backward direction. Polarization sensitive Monte Carlo simulations are used to verify some experimental observations. The results also demonstrate that surviving linear polarization fraction and overall intensity are more sensitive to the increase of glucose concentration in backward than in the forward direction in highly turbid media, indicating that backward geometry may be preferable for potential glucose detection in a biomedical context. Comparison measurements with optically inactive glycerol suggest that the refractive index matching effect, and not the chiral nature of the solute, dominates the observed optical rotation engendered by glucose in highly turbid media.
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