Deepa K. Kasaragod

Optic axis determination by fibre-based polarization-sensitive swept-source optical coherence tomography

We describe a fibre-based variable-incidence angle (VIA) polarization-sensitive swept-source optical coherence tomography (PS-SS-OCT) system to determine the 3D optical axis of birefringent biological tissues. Single-plane VIA-PS-OCT is also explored which requires measurement of the absolute fast-axis orientation. A state-of-the-art PS-SS-OCT system with some improvements both in hardware and software was used to determine the apparent optical birefringence of equine tendon for a number of different illumination directions. Polar and azimuthal angles of cut equine tendon were produced by the VIA method and compared with the nominal values. A quarter waveplate (QWP) and equine tendon were used as test targets to validate the fast-axis measurements using the system. Polar and azimuthal angles of cut equine tendon broadly agreed with the expected values within about 8% of the nominal values. A theoretical and experimental analysis of the effect of the sample arm fibre on determination of optical axis orientation using a proposed definition based on the orientation of the eigenpolarization ellipse experimentally confirms that this algorithm only works correctly for special settings of the sample arm fibre. A proposed algorithm based on the angle between Stokes vectors on the Poincaré sphere is confirmed to work for all settings of the sample arm fibre. A calibration procedure is proposed to remove the sign ambiguity of the measured orientation and was confirmed experimentally by using the QWP.

Experimental validation of an extended Jones matrix calculus model to study the 3D structural orientation of the collagen fibers in articular cartilage using polarization-sensitive optical coherence tomography

We report results to verify a theoretical framework to analyze the 3D depth-wise structural organization of collagen fibers in articular cartilage using polarization-sensitive optical coherence tomography. Apparent birefringence data obtained from multi-angle measurements using a time domain polarization-sensitive optical coherence tomography system has been compared with simulated data based on the extended Jones matrix calculus. Experimental data has been shown to agree with the lamellar model previously proposed for the cartilage microstructure based on scanning electron microscopy data. This tool could have potential application in mapping the collagen structural orientation information of cartilage non-invasively during arthroscopy.

Method to calibrate phase fluctuation in polarization-sensitive swept-source optical coherence tomography

We present a phase fluctuation calibration method for polarization-sensitive swept-source optical coherence tomography (PS-SS-OCT) using continuous polarization modulation. The method uses a low-voltage broadband polarization modulator driven by a synchronized sinusoidal burst waveform rather than an asynchronous waveform, together with the removal of the global phases of the measured Jones matrices by the use of matrix normalization. This makes it possible to average the measured Jones matrices to remove the artifact due to the speckle noise of the signal in the sample without introducing auxiliary optical components into the sample arm. This method was validated on measurements of an equine tendon sample by the PS-SS-OCT system.

Comparative study of the angle-resolved backscattering properties of collagen fibers in bovine tendon and cartilage

In a biological tissue, light scattering is based on the size and type of scatterers seen as refractive index variations that describe the optical properties shown. In this paper, we have implemented the variable incidence angle technique of multiple angle of illumination experiment on tendon and cartilage samples whose dominant constituents are genetically different types of collagen fibers, type I and type II, respectively. It is found that tendon displays a much greater angular anisotropy in its optical backscattering coefficient than the healthy cartilage. We propose that this is due to a more uniform distribution of fine fibrils than is found in tendon. Rayleigh-Gans approximation is used to give qualitative support to this idea.

Performance comparison between 8- and 14-bit-depth imaging in polarization-sensitive swept-source optical coherence tomography.

Recently the effects of reduced bit-depth acquisition on swept-source optical coherence tomography (SS-OCT) image quality have been evaluated by using simulations and empirical studies, showing that image acquisition at 8-bit depth allows high system sensitivity with only a minimal drop in the signal-to-noise ratio compared to higher bit-depth systems. However, in these studies the 8-bit data is actually 12- or 14-bit ADC data numerically truncated to 8 bits. In practice, a native 8-bit ADC could actually possess a true bit resolution lower than this due to the electronic jitter in the converter etc. We compare true 8- and 14-bit-depth imaging of SS-OCT and polarization-sensitive SS-OCT (PS-SS-OCT) by using two hardware-synchronized high-speed data acquisition (DAQ) boards. The two DAQ boards read exactly the same imaging data for comparison. The measured system sensitivity at 8-bit depth is comparable to that for 14-bit acquisition when using the more sensitive of the available full analog input voltage ranges of the ADC. Ex-vivo structural and birefringence images of equine tendon indicate no significant differences between images acquired by the two DAQ boards suggesting that 8-bit DAQ boards can be employed to increase imaging speeds and reduce storage in clinical SS-OCT/PS-SS-OCT systems. One possible disadvantage is a reduced imaging dynamic range which can manifest itself as an increase in image artifacts due to strong Fresnel reflection.

Speckle texture analysis of optical coherence tomography images

Optical coherence tomography (OCT) is an imaging technique based on the low coherence interferometry, in which signals are obtained based on the coherent addition of the back reflected light from the sample. Applying computational methods and automated algorithms towards the classification of OCT images allows a further step towards enhancing the clinical applications of OCT. One attempt towards classification could be achieved by statistically analyzing the texture of the noisy granular patterns - speckles that make the OCT images. An attempt has been made to quantify the scattering effects based on the speckle texture patterns the scatterers produce. Statistical inference is drawn from the textural analysis of the features based on the spatial intensity distribution on the agar phantoms with different concentration of Intralipid solutions. This preliminary study conducted on agar-Intralipid solution has showed us that it is possible to differentiate between different types of scatterers based on the speckle texture studies. The texture analysis has also been extended in an attempt to identify the invasion of melanoma cell into tissue engineered skin. However using the same approach of texture analysis, we have not obtained satisfactory results for carrying on with the computer-based identification of the invasion of the melanoma in the tissue engineered skin, the reason for which has to be further studied and investigated upon.

Photothermal detection of the contrast properties of polypyrrole nanoparticles using optical coherence tomography

We report on a photothermal modulation detection scheme developed using a swept source-based optical coherence tomography (OCT) system centred at 1300nm. Photothermal detection is an improved technique for studying the contrast properties of exogenous contrast agents such as highly absorbing polypyrrole (PPy) nanoparticles used for OCT imaging. The swept source based OCT system has a wavelength sweep rate of 10 kHz which is used for the phase modulation detection of various concentrations of PPy nanoparticles. PPy nanoparticles have been recently reported to be a promising candidate for OCT imaging owing to their strong NIR absorption from 700–1300nm. Phase-sensitive detection of the photothermal modulation signal is achieved using a pumped 975 nm laser beam at 80Hz and 160Hz for varying concentrations of PPy nanoparticles dispersed in 2% Intralipid phantom. A phase-sensitive detection system is realised by carrying out the phase calibration using the back reflections obtained from the coverslip used with the sample. This study provides quantitative support for the use of PPy nanoparticles as a potential biocompatible contrast agent in OCT imaging.

Depth-resolved dynamics of aceto-whitening in rabbit cornea studied by 1300nm optical coherence tomography

Cervical cancer is the eleventh most common cancer in the UK, especially for women under 35. In developed countries, cervical cancer is diagnosed by performing colposcopy. Contrast is enhanced by spraying dilute acetic acid onto the surface of the tissue. In the past decades, it has been shown that abnormal cervical epithelium turns opaque white upon contact with this weak acid whereas normal epithelium is generally not affected. This mechanism is known as aceto-whitening. However, the exact mechanism of this phenomenon is not fully known. In this study, OCT using near infrared light was used to quantify depth-resolved kinetics of aceto-whitening in a simple squamous epithelium model: rabbit cornea. We have found that both the epithelium and stroma brighten with approximately the same time course, reaching a peak reflectivity at about 50 seconds. The most significant increase in reflectivity was seen in the first 20 seconds upon the application of acid, and was measured to be 11dB. This result is compared with phosphate buffered saline solution, which was shown to exhibit no effect. Lactic acid, an alpha-hydroxy acid, has been reported as a negative control for aceto-whitening. However, our OCT results showed a significant epithelial brightening effect of approximately 8 dB in the first 20 seconds. The key difference with acetic acid is the lack of brightening in the corneal stroma. This could be due to inability to permeate through the basal lamina between corneal epithelium and stroma or lack of interaction with stromal keratocytes.

Determination of the collagen fiber ‘brushing direction’ in articular cartilage by conical-scan polarization-sensitive optical coherencetomography

A new imaging technique is presented by introducing the concept of conical scan to the variable-incidenc-angle polarimetry (VIA) previously developed by our group. The technique would facilitate the translating of the VIA technique to the clinic by simplifying the requirements of measurements in two orthogonal planes by using a conical scan protocol. Conical scan PS-OCT images could illustrate directly the azimuthal angle of the collage fibers in birefringent tissue, which was validated by measurements on a bovine tendon. We have showed the unique technique can be used to locate the “brushing direction” of collagen fibers in articular cartilage. Measuring this direction over the cartilage surface could potentially help designing of tissue-engineering scaffolds for cartilage repair.

Investigation of polarization-sensitive optical coherence tomography towards the study of microstructure of articular cartilage

This paper highlights the extended Jones matrix calculus based multi-angle study carried out to understand the depth dependent structural orientation of the collagen fibers in articular cartilage using polarization-sensitive optical coherence tomography (PS-OCT). A 3D lamellar model for the collagen fiber orientation, with a quadratic profile for the arching of the collagen fibers in transitional zone which points towards an ordered arrangement of fibers in that zone is the basis of the organization architecture of collagen fibers in articular cartilage. Experimental data for both ex-vivo bovine fetlock and human patellar cartilage samples are compared with theoretical predictions, with a good quantitative agreement for bovine and a reasonable qualitative agreement for human articular cartilage samples being obtained