Vanitha Sankaran

Comparison of polarized-light propagation in biological tissue and phantoms.

We demonstrate significant differences in the propagation of polarized light through biological tissue compared with two common tissue phantoms. Depolarization of linearly and circularly polarized light was measured versus propagation distance by use of two independent measurement techniques. The measurements were performed on adipose and myocardial tissues and on tissue phantoms that consisted of polystyrene microsphere suspensions and Intralipid. The results indicate that, in contrast with results obtained in tissue phantoms, linearly polarized light survives through longer propagation distances than circularly polarized light in biological tissue.

Comparative study of polarized light propagation in biologic tissues

We report the depolarization of light scattered by a variety of birefringent and nonbirefringent tissues. We used Stokes polarimetry to investigate how scatterer structures in each tissue contribute to the depolarization of linearly versus circularly polarized light propagating through that tissue. Experiments were performed on porcine blood, fat, tendon, artery, and myocardium. The results indicate that the two incident polarization states are depolarized differently depending on the structure of the sample. As seen in sphere suspensions, for tissues containing dilute Mie scatterers, circularly polarized light is maintained preferentially over linearly polarized light. For more dense tissues, however, the reverse is true. The results illustrate situations where polarized light will provide an improvement over unpolarized light imaging, information that is crucial to optimizing existing polarimetric imaging techniques.

Polarized light propagation through tissue phantoms containing densely packed scatterers.

We demonstrate that polarized light is maintained differently in densely packed versus dilute suspensions of polystyrene microspheres. The degrees of linear and circular polarization were measured versus scatterer concentration in aqueous suspensions of 0.48-, 0.99-, 2.092-, and 9.14-mum-diameter polystyrene microspheres. The results indicate that, for dilute suspensions of microspheres where independent scattering is assumed, the degrees of linear and circular polarization decrease as the scatterer concentration increases. For dense suspensions, however, the degree of polarization begins to increase as the scatterer concentration increases. The preferential propagation of linear over circular polarization states in dense suspensions is similar to results seen in biological tissue.

Polarization discrimination of coherently propagating light in turbid media.

We describe the use of degree of polarization to discriminate unscattered and weakly scattered light from multiply scattered light in an optically turbid material. We use spatially resolved measurements of the degree of polarization to compare how well linearly and circularly polarized light survives in a sample. Experiments were performed on common tissue phantoms consisting of polystyrene and Intralipid microsphere suspensions and on adipose and arterial tissue. The results indicate that polarization is maintained even after unpolarized irradiance through each sample has been extinguished by several orders of magnitude. The results also show that polarized light propagation in common tissue phantoms is distinctly different from polarized light propagation in the two tissues investigated. Further, these experiments illustrate when polarization is an effective discrimination criterion and when it is not. The potential of a polarization-based discrimination scheme to image through the biological and nonbiological samples investigated here is also discussed.

Birefringence measurement of rapid structural changes during collagen denaturation.

Abstract— Linear birefringence, an optical property that results from a materials structure and composition, can be used to study dynamic changes in tissue structure. Single, 200 μs‐long pulses from a Ho:YAG laser emitting 2.1 μm radiation were used to induce changes in the linear birefringence of rat tail tendon. Such changes were measured on a millisecond timescale. The measured rate coefficients describing the denaturation are not predicted by previous studies of collagen denaturation induced by slower, lower‐temperature heating. Two types of laser‐induced collagen denaturation can be differentiated: thermal denaturation, which appears rate‐limited, and thermomechanical denaturation, which is observed at higher laser radiant exposures. Neither process is described by standard Arrhenius‐type kinetic models.

Polarized light propagation in turbid media

Polarimetry, which is a comparison of the polarization state of light before and after it has interacted with a material, can be used to discriminate unscattered and weakly scattered photons from multiply scattered photons. Weakly scattered photons tend to retain their incident polarization state whereas highly scattered photons become depolarized; thus, polarization-based discrimination techniques can be used to image through tissue with decreased noise and increased contrast. Many previous studies investigating polarization- based discrimination have been conducted on tissue phantoms, with the ultimate goal being noninvasive imaging of breast tumors. We demonstrate here that linearly and circularly polarized light propagate differently in common tissue phantoms than in two independent techniques on tissue phantoms consisting of polystyrene and Intralipid microsphere suspensions, and on porcine adipose tissue and porcine myocardium. We show that contrary to expectations made from studies in the phantoms, linearly polarized light survives through more scattering events than circularly polarized light in both adipose tissue, which contains quasi-spherical scatterers, ad myocardium, which contains quasi-spherical and cylindrical scatterers. Differences between spherical and biological scatterers are discussed, along with the impact of tissue birefringence on degree of polarization measurements.

Optical real-time measurement of collagen denaturation

Linear birefringence is a property of collagenous tissue that results from both its composition and structure. Previous investigations have shown that birefringence provides an indication of structural changes in collagen during slow heating. We now report the birefringent response of both mature and young rat tail tendon to laser-heating. The results indicate that denaturation of collagen from mature rats induced by a 200-microsecond(s) -long Ho:YAG laser pulse may not be described accurately by kinetic parameters. Several second-long pulses of CO2 laser pulse may not be described from young rats fit an Arrhenius model with Ea equals 12.1 kcal/mol and A equals e18.03 s-1. Typically, for slow-heating of collagen, Ea equals kcal/mol and A equals e120 s-1. Thus, it seems likely that the temperature and energy needed to initiate collagen denaturation is lower in young collagen, possibly due to its decreased hydroxyproline content and consequent decreased thermal stability.

Polarized light propagation in biologic tissue and tissue phantoms

Imaging through biologic tissue relies on the discrimination of weakly scattered from multiply scattered photons. The degree of polarization can be used as the discrimination criterion by which to reject multiply scattered photons. Polarized light propagation through biologic tissue is typically studied using tissue phantoms consisting of dilute aqueous suspensions of microspheres. We show that, although such phantoms are designed to match the macroscopic scattering properties of tissue they do not accurately represent biologic tissue for polarization-sensitive studies. In common tissue phantoms, such as dilute Intralipid and dilute 1-micrometers -diameter polystyrene microsphere suspensions, we find that linearly polarized light is depolarized more quickly polarized light. In dense tissue, however, where scatterers are often located in close proximity to one another, circularly polarized light is depolarized similar to or more quickly than linearly polarized light. We also demonstrate that polarized light propagates differently in dilute versus densely packed microsphere suspensions, which may account for the differences seen between polarized light propagation in common dilute tissue phantoms versus dense biologic tissue.

Performance assessment of the SPDI subsurface imaging technique

Summary form only given.The ability of the spectral and polarization difference imaging (SPDI) technique to provide deep subsurface imaging in tissues is evaluated. The SPDI technique utilizes the wavelength dependence of the mean visit depth of photons inside a tissue sample before they emerge in the backscattered direction. Polarization filtering is used to discriminate against reflected photons from the superficial tissue layers. In addition, this technique involves subtraction of the perpendicular polarization image components obtained under polarized illumination at different wavelengths following appropriate adjustment of the time of exposure in order to normalize the number of photons backscattered from the outer tissue layers. This operation leads to a new image arising from photons that propagated deeper into the tissue which are contained in larger numbers in the image obtained using the longer illumination wavelength.

Polarized light propagation through tissue and tissue phantoms

We show that standard tissue phantoms can be sued to mimic the intensity and polarization properties of tissue. Polarized light propagation through biologic tissue is typically studied using tissue phantoms consisting of dilute aqueous suspensions of microspheres. The dilute phantoms can empirically match tissue polarization and intensity properties. One discrepancy between the dilute phantoms and tissue exist: common tissue phantoms, such as dilute Intralipid and dilute 1-micrometers -diameter polystyrene microsphere suspension, depolarize linearly polarized light more quickly than circular polarized light. In dense tissue, however, where scatterers are often locate din close proximity to one another, circularly polarized light is depolarized similar to more quickly than linearly polarized light. We also demonstrate that polarized light is depolarized similar to or more quickly than linearly polarized light. We also demonstrate that polarized light propagates differently in dilute versus densely packed microsphere suspensions, which may account for the differences seen between polarized light propagation in common dilute tissue phantoms versus dense biologic tissue.