H. S. Patel

Depolarization of light in tissue phantoms - effect of a distribution in the size of scatterers.

We show that the depolarization behavior of light on propagation through a sample having a mixture of suspension of monodisperse polystyrene microspheres of two different sizes (mean diameter 0.11microm and 1.08 microm) is dominated by the smaller of the two scatterers. In contrast the estimates for the anisotropy parameter (g) for this sample, obtained from goniophotometric measurement, are observed to be closer to the value corresponding to the larger of the two scatterers. These results imply that the depolarization behavior of light in biological tissue (having a distribution of scatterer size) would be different from that of a matched monodisperse scattering sample having the same value of anisotropy parameter (g) and optical thickness (tau = micros x d, micros is scattering coefficient and d being the physical thickness).

Depth-resolved fluorescence measurement in a layered turbid medium by polarized fluorescence spectroscopy.

We show that, when a turbid medium with a layered fluorophore distribution is excited by linearly polarized light, measurement of angle-resolved polarized fluorescence can provide depth-resolved fluorescence measurements.

Depolarization of light in tissue phantoms: effect of collection geometry

Abstract We report measurement of depolarization of light on propagation through matched scattering samples having the same anisotropy parameter (g) and optical thickness (τ=μs×d, d being the physical thickness) but different values for scattering coefficient (μs). Significant differences were observed in the depolarization behavior of the samples so matched. The depolarization of both linear and circularly polarized light was always larger for the sample with higher value of μs. Further, for lower value of g (g∼0.1), the difference in depolarization between matched samples was larger for circularly polarized light than that for linearly polarized light. Exactly the opposite result was obtained for matched samples with larger g values (g∼0.9). We show that these differences in the depolarization behavior of matched samples are dependent on the collection geometry and arise because for a given collection angle, a greater fraction of multiply scattered photons are collected for the sample having higher μs.

Experimental investigation of perturbation Monte-Carlo based derivative estimation for imaging low-scattering tissue

Experimental results for imaging the low-scattering tissue phantoms based on the derivative estimation through perturbation Monte-Carlo (pMC) method are presented. It is proven that pMC-based methods give superior reconstructions compared to diffusion-based reconstruction methods. An easy way to estimate the Jacobian using analytical expression obtained from perturbation Monte-Carlo method is employed. Simulation studies on the same objects, considered in the experiment, are performed and corresponding results are found to be in reasonable agreement with the experimental studies. It is shown that inter-parameter cross talk in diffusion based methods lead to false results for the low-scattering tissue, where as the pMC-based method gives accurate results.

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.

Simultaneous determination of size and refractive index of red blood cells by light scattering measurements

We present a fast Fourier transform-based approach for simultaneous determination of size and refractive index of a collection of red blood cells from the measured angular distribution of scattered light.

Time-gated optical imaging through turbid media using stimulated Raman scattering: Studies on image contrast

In this paper, we report the development of experimental set-up for time-gated optical imaging through turbid media using stimulated Raman scattering. Our studies on the contrast of time-gated images show that for a given optical thickness, the image contrast is better for sample with lower scattering coefficient and higher physical thickness, and that the contrast improves with decreasing value of anisotropy parameters of the scatterers. These results are consistent with time-resolved Monte Carlo simulations.

Biomedical Applications of Lasers

Lasers are being increasingly used for biomedical imaging, diagnosis, and therapy. In this chapter we first provide a brief overview of light propagation in tissue. Next we discuss the techniques developed for a more comprehensive utilization of the information content of the light scattered from tissue and how this is helping high-resolution biomedical imaging of tissue microstructure and quantitative, sensitive, and noninvasive diagnosis. The use of lasers for ultraprecise surgery in a minimally invasive manner and also the use of photoactivated drugs for therapy with high selectivity are discussed next. By using illustrations from the work carried out at RRCAT, the chapter also provides a brief overview of the activities at RRCAT in these areas.

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.