M. K. Swami

Polar decomposition of 3×3 Mueller matrix: a tool for quantitative tissue polarimetry

The polarization properties of any medium are completely described by the sixteen element Mueller matrix that relates the polarization parameters of the light incident on the medium to that emerging from it. Measurement of all the elements of the matrix requires a minimum of sixteen measurements involving both linear and circularly polarized light. However, for many diagnostic applications, it would be useful if the polarization parameters can be quantified with linear polarization measurements alone. In this paper, we present a method based on polar decomposition of Mueller matrix for quantification of the polarization parameters of a scattering medium using the nine element (3 x 3) Mueller matrix that requires linear polarization measurements only. The methodology for decomposition of the 3 x 3 Mueller matrix is based on the previously developed decomposition process for sixteen element (4 x 4) Mueller matrix but with an assumption that the depolarization of linearly polarized light due to scattering is independent of the orientation angle of the incident linear polarization vector. Studies conducted on various scattering samples demonstrated that this assumption is valid for a turbid medium like biological tissue where the depolarization of linearly polarized light primarily arises due to the randomization of the field vectors direction as a result of multiple scattering. For such medium, polar decomposition of 3 x 3 Mueller matrix can be used to quantify the four independent polarization parameters namely, the linear retardance (delta ), the circular retardance (psi), the linear depolarization coefficient (Delta) and the linear diattenuation (d) with reasonable accuracy. Since this approach requires measurements using linear polarizers only, it considerably simplifies measurement procedure and might find useful applications in tissue diagnosis using the retrieved polarization parameters.

Polarized diffuse reflectance measurements on cancerous and noncancerous tissues

We report the measurement of polarization parameters (linear retardance, diattenuation and depolarization) of normal and malignant tissue from human oral cavity and breast over the spectral range 390 nm to 550 nm. These parameters were determined using the 3 x 3 Mueller matrix, the elements of which could be determined using linear polarization measurements only. The significant differences observed in the polarization parameters of the normal and malignant tissues appear to arise because of the changes in the collagen matrix in the two tissue types.

Imaging of human breast tissue using polarization sensitive optical coherence tomography

We report a study on the use of polarization sensitive optical coherence tomography (PSOCT) for discriminating malignant (invasive ductal carcinoma), benign (fibroadenoma) and normal (adipocytes) breast tissue sites. The results show that while conventional OCT, that utilizes only the intensity of light back-scattered from tissue microstructures, is able to discriminate breast tissues as normal (adipocytes) and abnormal (malignant and benign) tissues, PS-OCT helps in discriminating between malignant and benign tissue sites also. The estimated values of birefringence obtained from the PSOCT imaging show that benign breast tissue samples have significantly higher birefringence as compared to the malignant tissue samples.

Mueller matrix measurements on absorbing turbid medium

Polarization parameters of diffuse backscattered light from a turbid sample are sensitive to its structural properties and can, therefore, be used to probe morphological features of tissue and, thus, monitor changes that arise due to a disease. Extraction of morphological information from measured polarization parameters, however, requires a careful understanding of the dependence of these on factors such as size, size distribution, shape, and dielectric constant of the scatterers, which are often quite involved. In particular, the presence of absorption complicates the dependence of polarization parameters on tissue morphological features. We have found that, while for medium comprising small size scatterers (Rayleigh scatterers), the depolarization shows the expected decrease with an increase in the absorption of the scattering medium, a counterintuitive behavior was observed for larger size (>lambda) scatterers. Further analysis of the results suggests that the observed behavior might arise due to the relative contribution of two depolarizing processes, one resulting from a series of out-of-plane scattering and the other due to the angular variation of the state of polarization in a single scattering event.

Effect of gold nanoparticles on depolarization characteristics of Intralipid tissue phantom

We report results of studies on the effect of different shapes and sizes of gold nanoparticles (GNPs) on the depolarization characteristics of Intralipid tissue phantoms. For a given extinction coefficient, the absorption characteristics of the GNPs contributed more to the depolarization of the turbid medium, and the contribution of scattering was significant only for the larger-sized particles. For rod-shaped GNPs, their depolarization plays an important role in the low scattering regime in which the depolarization properties of the nanoparticles (NPs) dominate in the turbid medium even if the contribution of the scattering from the NPs is about an order of magnitude less. For highly scattering samples, GNP absorption significantly modulates the depolarization spectra of the turbid medium.

Size-dependent patterns in depolarization maps from turbid medium and tissue.

Mueller matrix measurements on turbid media can be used to quantify its polarization properties in terms of retardance, diattenuation, and depolarization. In particular, the depolarizing ability of such media, which is represented by the depolarization index, has been shown to be a useful diagnostic parameter. However, being a single valued metric, its dependence on a host of tissue optical parameters makes it difficult to interpret. In this paper, we show that a map of depolarization as a function of input polarization state parameters can be used to infer information about the size of scatterer and order of birefringent and depolarizing layers in turbid medium. The experiments carried out on different mice organ tissues indicate that the depolarization characteristics of tissue are closely represented by depolarization properties of intralipid. We also observed that these maps do not vary in the presence of absorption.

Photonic nanojet assisted enhancement of Raman signal: Effect of refractive index contrast

We discuss the enhancement of Raman signals using a photonic nanojet generated by dielectric microspheres. The highly confined field of the nanojet leads to an order of magnitude enhancement of the Raman signal from the substrate beneath. Enhancement is observed to depend strongly on the sizes of the microspheres as well as the contrast between their refractive index and that of the sample. Enhancement increases when the refractive index of the substrate increases relative to that of the microsphere, but decreases rapidly as the two become equivalent.

Effect of surface interactions on diffusion of molecules: Fluorescence correlation spectroscopic investigation

Fluorescence correlation spectroscopy study showed that apart from its surface charge, the hydrophobicity of the substrate significantly influences the diffusion of the molecules and must be taken into account for a correct interpretation of the FCS data.

Mueller Matrix Approach for Discriminating Optical Rotation

The vector nature of light carries information that enables the non-invasive measurement of glucose in a turbid medium: blood.