K. Divakar Rao

Depth‐sensitive Raman spectroscopy combined with optical coherence tomography for layered tissue analysis

Complete characterization of a layered tissue requires probing both the biochemical and the morphological information from its different layers at various depths. We report the development of a combined Raman spectroscopy (RS) and optical coherence tomography (OCT) system that is capable of measuring depth-sensitive Raman signal from the tissue layers imaged by the OCT. The sample arm of a real-time time-domain OCT system was modified to allow for co-alignment of the OCT with the Raman probe beam. The depth sensitivity of Raman was obtained by incorporating confocal Raman configuration that minimized out-of-focus Raman scattered light. The system was first validated using a layered phantom prepared by depositing a thin layer of paraffin over acetaminophen. A good correlation was observed between the OCT images and the Raman signal. The system was also used to record OCT and Raman images of a resected mucosal tissue sample. While OCT image showed the presence of epithelial and stromal layers, Raman spectra measured from these layers confirmed the biochemical difference between the two.

Real-time in vivo imaging of adult Zebrafish brain using optical coherence tomography

We report noninvasive imaging of the brain of adult zebrafish (Danio rerio) using real time optical coherence tomography (OCT) capable of acquiring cross sectional 2D OCT images @ 8 frames/sec. Anatomic features such as telencephalon, tectum opticum, eminentia Granularis and cerebellum were clearly resolved in the OCT images. A 3D model of zebrafish brain was reconstructed, for the first time to our knowledge, using these 2D OCT images.

Use of common path phase sensitive spectral domain optical coherence tomography for refractive index measurements

We report the use of a common path phase sensitive spectral domain optical coherence tomography setup for the measurement of the refractive index (RI) of a biomimetic material (glucose solution in water having intralipid as the scattering medium) and a single biological cell (keratinocyte). The RI of glucose solutions could be measured with a precision of ∼0.00015, which corresponds to a precision of ∼2 nm in the optical path length measurement in our setup. The precision obtained in the measurement of the RI of a single keratinocyte cell was ∼0.0004.

BINARY TISSUE CLASSIFICATION STUDIES ON RESECTED HUMAN BREAST TISSUES USING OPTICAL COHERENCE TOMOGRAPHY IMAGES

We report the results of a comparative study of Fourier domain analysis (FDA) and texture analysis (TA) of optical coherence tomography (OCT) images of resected human breast tissues for binary classification between normal–abnormal classes and benign–malignant classes. With the incorporation of Fisher linear discriminant analysis (FLDA) in TA for feature extraction, the TA-based algorithm provided improved diagnostic performance as compared to the FDA-based algorithm in discriminating OCT images corresponding to breast tissues with three different pathologies. The specificity and sensitivity values obtained for normal–abnormal classification were both 100%, whereas they were 90% and 85%, respectively for benign–malignant classification.

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.

Noninvasive imaging of ethanol-induced developmental defects in zebrafish embryos using optical coherence tomography.

In this article, we report the use of optical coherence tomography for noninvasive cross-sectional real-time imaging of ethanol-induced developmental defects in zebrafish embryos larvae. For ethanol concentration of over 300 mM, developmental defects of eye (shrinkage and retinal abnormalities), malformation of the notochord and ataxia arising due to the toxic effects of ethanol were observed in OCT images from 3 days post fertilization onwards. The results suggest that OCT could be a valuable tool for noninvasive assessment of birth defects in small animal systems.

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.

Surface-plasmon-resonance enhancement: effects on optical trapping and manipulation of nano-objects

The utilization of the enhanced local field near trapped metallic nanoparticles due to surface-plasmon resonance (SPR) for the optical trapping of dielectric fluorescent nano-objects is of considerable interest for single-molecule manipulation. Theoretical calculations as well as experimental measurements show that even with moderate SPR based field enhancement factors, gradient force based trapping of fluorescent molecules would be rather difficult. While trapping of the fluorescent molecule at resonance wavelength showed decreased stiffness, at wavelengths far away from resonance, increase in stiffness was found which was attributed to interplay of SPR-enhanced absorption and gradient forces.

In-Vivo Imaging Of Adult Zebrafish Using Optical Coherence Tomography

Zebrafish (Danio rerio), a vertebrate, is a good model system in medical research to understand a variety of human biological processes. These studies often require measurements of the morphological and physiological parameters of Zebrafish. Therefore, development of noninvasive imaging techniques for this purpose is of considerable interest. In this article, we present a brief overview of the use of OCT for noninvasive in-vivo imaging of adult Zebrafish.

Tapered single mode fiber tip for high lateral resolution imaging in optical coherence tomography

We report the use of a chemically etched tapered single mode fiber tip for enhancing lateral resolution in optical coherence tomography (OCT). The important advantage of this approach is that high lateral resolution is achieved, without compromising the depth of imaging, as is the case with the use of high numerical aperture (NA) objectives. Use of the tapered tip in the sample arm of a single mode fiber based set-up allowed visualization of intracellular structures of Elodea densa plant leaf that could not be seen by the conventional OCT.