Investigation into swept source based optical coherence tomography for bio-imaging
Meleppat, Ratheesh Kumar
Date of Issue2017-02-27
School of Mechanical and Aerospace Engineering
Optical imaging at both the macroscopic and microscopic levels is used intensively these days by clinicians for diagnosis and treatment. New advances in optics, data acquisition, and signal processing enabled the development of novel optical imaging technologies with enhanced imaging capabilities for targeted applications. Further, optical imaging technologies are more affordable than conventional radiation based technologies and provide both structural and functional information with enhanced resolution. Among different optical imaging modalities, optical coherence tomography (OCT) has gained a special attraction since it allows a non-invasive and in-vivo subsurface imaging of the biological specimen at high resolution and relatively better penetration depth. Fourier-domain OCT (FDOCT) configurations are highly attractive for many diagnostic imaging applications due to their potential for high speed imaging with high sensitivity. Nevertheless, the imaging capability and applicability of OCT are often challenged by the factors such as complex and delicate system design, degradation of axial resolution and sensitivity, and reduced contrast. Accordingly, intense efforts aimed at making technical improvements in resolution, image quality, and functional capabilities are underway among the scientific community. A compact FDOCT system based on swept-source configuration (SSOCT) with enhanced imaging capabilities has been proposed and demonstrated in this thesis. An interferometry system based on the non-reflective reference arm has been introduced in place of the Michelson interferometer to offer highly stable, alignment free and portable architecture. The degradation of the axial resolution and sensitivity caused by the non-linear sweeping characteristics of the laser and optical dispersion are efficiently corrected by a spectral phase-based wavenumber calibration and a numerical dispersion correction scheme, respectively. The proposed calibration scheme is implemented by an automatic method, which further reduces the computational and hardware complexity. Performance of the system is analyzed experimentally and compared with their theoretically predicted values. Contrast enhancement in OCT imaging using plasmonic nanoprobes has been investigated. Plasmon-resonant nanostructures of gold and silver have been demonstrated as powerful contrast agents for OCT imaging around 1300 nm. The localized surface plasmon response (LSPR) induced extinction properties of the nanoparticles have been characterized by OCT using a cross-correlation approach. The extinction cross-section measured using OCT found to be consistent with the simulation model based on finite difference time domain (FDTD) and spectroscopy method. The qualitative and quantitative investigations of the contrast enhancement using proposed nanoprobes have been conducted on both biological tissue as well as tissue mimicking phantom. The project further aims in exploring the potential of optical coherence tomography and microscopy for the real-time in-situ investigation of bacterial biofilms. Both the cross-sectional and enface images are presented to study the development of biofilms and their time-resolved growth. The simultaneous measurement of refractive index and the thickness of the biofilms (Pseudomonas Aeruginosa) using OCT are demonstrated. Monitoring of growth dynamics of biofilm (Klebsiella Pneumonia) using optical density (OD) and planar density are also demonstrated using optical coherence microscopy technique and compared with OD and colony forming units (CFUs) measured using standard procedures. It is envisaged that the outcome of the research presented in this thesis will contribute well towards the diagnostic abilities of the OCT based bio imaging systems.