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.
The effects of optically turbid medium on polarization states of incident light are studied using a novel linear Stokes polarimeter. The optical rotation and surviving linear polarization fraction of light scattered from highly turbid media (scattering coefficient μs = 100 cm-1) are measured both in and off the incident plane while the detection angle changes from forward direction (0°) to backward directions (135°, 145° and 155°). The response of the optical rotation and surviving linear polarization to the presence of glucose molecules (0.06M - 0.9M) is also studied. The results show that in the absence of glucose, the scattering-induced optical rotation is zero in the incident plane for all detection angles, and increases with detection angle when measured off the incident plane. Conversely, the surviving linear polarization fraction increases with detection angle in the incident plane, and decreases when off the incident plane. Thus, when measured in the incident plane, optical rotation is least sensitive to glucose in the turbid medium, whereas the surviving linear polarization is most sensitive. For the above turbidity and glucose concentration ranges, the optimal glucose detection sensitivity using optical rotation is at 135° detection angle, 2 mm off the incident plane, while it is at 135° detection angle in the incident plane if surviving linear polarization is used as a glucose probe. This work demonstrates the complexity of polarimetry in turbid chiral media and underscores the importance of detection geometry in making and interpreting turbid polarimetry measurements.
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