Sujatha Narayanan Unni

Understanding the changes in microcirculatory flow and its measurements are very important for assessing the progress of various vascular malfunctions and their subsequent treatment effectiveness. Laser Speckle Contrast Imaging (LSCI) has been evolved as a whole-field, non-invasive and non-contact technique which has inherent advantages for microcirculation assessment in an in vivo environment compared to its noninvasive counterparts such as laser Doppler technique and video capillaroscopy. However, representation of flow velocity values in absolute units is still challenging and yet to be completely explored. In this paper, we propose an experimental model for estimating the flow velocity based for optimum camera exposure time. The LSCI experiments were conducted on a custom made phantom flow channel with induced flow in the microcirculation range using a syringe pump. The speckle image contrast was estimated temporally and is used to calculate velocity values. The relative error in the flow values is estimated to be a function of the calculated contrast. The estimated error has been incorporated as a correction factor in the obtained velocity term using LSCI and final velocity estimation was found to be within an acceptable error range independent of the flow velocity and scatterer concentration of the sample for optimum camera exposure duration.

Fluorescence from endogenous fluorophores has been emerging as a promising biomarker for tissue discrimination resulting a noninvasive screening methodology to understand the biochemical and morphological variations in tissues associated with cancer development. We have developed a scan based fiber optic probe system to image increased flavin adenine dinucleotide (FAD) fluorescence from epithelial tissues under conditions mimicking dysplasia surrounded by normal tissues. Experiments were conducted on optical phantoms mimicking epithelial tissues excited by 450nm LED source. The spectral emission from the sample is collected via optical fibers and the imaging is performed by scanning the sample using a translation stage at desired resolution. Monte Carlo simulations were also performed by devising an optical model corresponding to epithelial tissue and the results were correlated with experimental fluorescence measurements. This whole field imaging approach could be useful for in vivo assessment of tissue pathologies based on auto fluorescence and can give a better quantitative approach for estimation of tissue properties by correlating the experimental and simulated data.

In biomedical optical spectroscopy tissue-mimicking phantoms have been widely used for imitating optical properties of biological tissues. As tissue is a turbid medium involving scatterers, absorbing and fluorescing molecules, modelling a tissue in the form of a phantom should have the same realistic complexity comparable to that of tissues. In optical spectroscopy, fluorescence phenomena have been extensively investigated as an optical technique for disease diagnosis. The fluorescence signal is distorted by optical properties of a biological tissue. The purpose of this study is to investigate whether the use of Intralipid as a scattering agent in a turbid medium containing fluorophores can affect fluorescent intensity by the phenomena of scattering and collisional quenching. The results indicate that phantom sets with different concentrations of Tyrosine and Intralipid have their emission peaks distorted at 300 nm and also show secondary peaks when used for fluorescence studies in UV region. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Dilutions of Intralipid 20% are widely used as optical phantoms for mimicking scattering properties of turbid media such as tissues. One of the frequently used methodologies for quantifying the scattering coefficient and anisotropy of Intralipid 20% is the use of single-particle Mie scattering theory, which in fact is not valid for nontenuous media. Hence, two methodologies consisting of analytical wave theory and effective medium theory, incorporating particle size distribution and concentration-dependent correlated scattering phenomena, are used to estimate the effective scattering coefficient and anisotropy of Intralipid 20% dilutions (1%-100% v/v) from 380 to 1000 nm.

This work presents a combination of differential absorption technique and frequency domain optical coherence tomography for detection of glucose, which is an important analyte in medical diagnosis of diabetes. Differential absorption technique is used to detect glucose selectively in the presence of interfering species especially water and frequency domain optical coherence tomography (FDOCT) helps to obtain faster acquisition of depth information. Two broadband super-luminescent diode (SLED) sources with centre wavelengths 1586 nm (wavelength range of 1540 to 1640 nm) and 1312 nm (wavelength range of 1240 to 1380 nm) and a spectral width of 60nm (FWHM) are used. Preliminary studies on absorption spectroscopy using various concentrations of aqueous glucose solution gave promising results to distinguish the absorption characteristics of glucose at two wavelengths 1310 nm (outside the absorption band of glucose) and 1625 nm (within the absorption band of glucose). In order to mimic the optical properties of biological skin tissue, 2% and 10% of 20% intralipid with various concentrations of glucose (0 to 4000 mg/dL) was prepared and used as sample. Using OCT technique, interference spectra were obtained using an optical spectrum analyzer with a resolution of 0.5 nm. Further processing of the interference spectra provided information on reflections from the surfaces of the cuvette containing the aqueous glucose sample. Due to the absorption of glucose in the wavelength range of 1540 nm to 1640 nm, a trend of reduction in the intensity of the back reflected light was observed with increase in the concentration of glucose.

Light-based diagnostic techniques provide a minimally invasive way for selective biomarker estimation when tissues transform from a normal to a malignant state. Spectroscopic techniques based on diffuse reflectance characterize the changes in tissue hemoglobin/oxygenation levels during the tissue transformation process. Recent clinical investigations have shown that changes in tissue oxygenation and microcirculation are observed in diabetic subjects in the initial and progressive stages. In this pilot study, we discuss the potential of diffuse reflectance spectroscopy (DRS) in the visible (Vis) range to differentiate the skin microcirculatory hemoglobin levels between normal and advanced diabetic subjects with and without neuropathy. Average concentration of hemoglobin as well as hemoglobin oxygen saturation within the probed tissue volume is estimated for a total of four different sites in the foot sole. The results indicate a statistically significant decrease in average total hemoglobin and increase in hemoglobin oxygen saturation levels for diabetic foot compared with a normal foot. The present study demonstrates the ability of reflectance spectroscopy in the Vis range to determine and differentiate the changes in tissue hemoglobin and hemoglobin oxygen saturation levels in normal and diabetic subjects.

Laser Doppler Blood Flow meter uses tissue backscattered light to non-invasively assess the blood flow rate. qualitatively. As there is large spatial variability and the temporal heterogeneity in tissue microvasculature, the measured blood flow rate is expressed in relative units. A non-linear approach in order to understand the dynamics of the microcirculation led to the fractal characterization of the blood flow signals. The study presented in the paper aims to analyze the fractal behavior of Laser Doppler Flow (LDF) signals and to quantitatively estimate the fractal dimension of waveforms using Box-Counting method. The measured Fractal dimension is an estimate of temporal variability of tissue perfusion. The rate at which fractal dimension varies as a function of location between individuals, exhibits a weak correlation with time. Further studies with a larger number of subjects are necessary to test the generality of the findings and if changes in dimension are reproducible in given individuals. In conclusion, the fractal dimension determined by Box-counting method may be useful for characterizing LDF time series signals. Future experiments evaluating whether the technique can be used to quantify microvascular dysfunction, as commonly occurring in conditions such as Diabetes, Raynaud's phenomenon, Erythromelalgia and Achenbach syndrome needs to be evaluated. © 2011 Published by Elsevier B.V.

Quantitative Mueller polarimetry optically characterizes a medium and is reflected upon by the ultrastructural changes in it. Tissue morphology changes occur during advent of diseases like cancer neoplasia. This alters the Mueller matrix characterizing the tissue as an optical element. The nucleus size undergoes an approximate doubling during the development of cancer. Cell crowding during cancer increases the number density of the nuclei per unit volume. Modeling the cell nuclei as main scattering centers, a systematic computational study on how Mueller matrix elements vary for an increase in scatterer size and number density is performed. Simulation on polarized light transport of wavelength 633nm through a slab of size 3 mm comprising of spherical scatterers in a medium of refractive index 1.33 is carried out. Light propagation is modeled using Monte Carlo method and meridian plane method is adopted for tracking the polarization state change. The stokes vector of the outgoing light is tracked to calculate the Mueller matrix images of the light backscattered from the slab. The Mueller matrix elements as well as depolarization factors are derived. The depolarization index increases with scatterer size. Along with nucleus size, change in the cell number density is also expected in the different stages of the cancer growth. Volume fraction of the scatterers in medium is varied as an indicator of this number density change. Behavior of Mueller matrix with respect to change in scattering coefficient due to variation in scatterer size and volume fraction is studied. It is observed that the depolarization index derived from Mueller matrix has selective discrimination towards the change in scattering coefficient caused due to size change and volume fraction change respectively.

Measurement and analysis of microcirculation is vital in assessing local and systemic tissue health. Changes in microvascular perfusion if detected can provide information on the development of various related diseases. Laser Doppler blood flowmetry (LDF) provides a non-invasive real-time measurement of cutaneous blood perfusion. LDF signals possess fractal nature that represents the correlation in the successive signal elements. Changes in the correlation of flow and its associated parameters could be used as a tool in differentiating the ailments at different stages or assessing the treatment effectiveness of a particular ailment. Spectral domain analysis of LDF signals reveals five characteristic frequency peaks corresponding to local and central regulatory mechanisms of the human body, namely metabolic, neurogenic, myogenic, respiration, and heart rate. This paper investigates the changes in the fractal nature and constituent frequency bands of laser Doppler signals in diabetic and healthy control subjects acquired from the glabrous skin of the foot so as to provide an assessment of microcirculatory dynamics. As a pilot study, it was attempted on a set of healthy control and diabetic volunteers, and the obtained results indicate that fractal nature of LDF signals is less in diabetic subjects compared to the healthy control. The wavelet analysis carried out on the set of signals reveals the dynamics of blood flow which may have led to the difference in correlation results.

Supercontinuum source based Differential Absorption Optical Coherence Tomography (DAOCT) technique in near infrared (1.3 to 2.3 μm) region is proposed and demonstrated with aqueous glucose samples for enhanced selectivity and specificity of glucose detection.