Endoscopic Optical Coherence Tomography And Its Application On Upper Gastrointestinal Tract

Optical coherence tomography(OCT)is a high-resolution,non-invasive,label-free in-vivo imaging technique that uses optical coherence gate to provide depth-resolved structures of biological tissues.In most highly scattering tissues,OCT can penetrate 1 to 3 mm at micronlevel resolution.With almost 30 years rapidly development,the imaging speed of OCT hitherto is now fast enough to make real-time two-dimensional and in vivo three-dimensional imaging possible.The contrast of common OCT images originates from the refractive index change of tissues.Images obtained from samples with small refractive index changes have poor contrast,resulting in a system even with high resolution cannot reveal structures clearly.Besides,the minimized fiber optic-based endoscope is not suitable for upper digestive tract imaging because of the poor maneuverability and short working distance.In light of the above two shortcomings,based on Fourier Domain OCT system,this dissertation studies the endoscopic OCT technology from two aspects: OCT endoscopes and quantitative analysis of OCT data.The achieved innovations are summarized as following:1.A robust,depth-resolved optical attenuation coefficient calculation algorithm for OCT data was proposed.By taking full advantage of the depth-resolved ability of OCT and optical property differences between tissue structures,attenuation coefficient image that retains the localized information is achieved from the original cross-sectional intensity image as a contrast complement.After analyzing the attenuation of the OCT signal and the noise floor,the new algorithm simultaneously corrects: 1)overestimation of the attenuation coefficient of the deep tissue caused by the incomplete attenuation of the incident light;2)due to the existence of the noise floor,OCT data violates the ideal exponential decay model and underestimates the attenuation coefficient of shallow tissues.The algorithm affords consistently high accuracy over the entire imaging depth for practical biological samples of different scattering properties.2.A high-speed endoscopic swept source OCT system operating at 1300 nm wavelength region was developed.In view of the complex topology of the oral and maxillofacial tissues,a low-cost,compact handheld OCT endoscopic probe was designed and fabricated.A dualaxis microelectromechanical system(MEMS)is deployed at the proximal end of the probe and relays the scanned beam to the distal end through a 4f configuration to achieve dualaxis telecentric scan.This design not only allows the use of a slightly larger MEMS scanner for better optical and mechanical performance,but also ensures MEMS that require highvoltage drive are kept away from the tissue being interrogated,reducing the risk of electric shocked.The slender lens set design provides the probe excellent maneuverability and thus easy access of deep oral cavity.3.An ultra-high resolution endoscopic spectral domain OCT system operating at 800 nm wavelength region.For investigating of the tubular-shaped esophagus,a super-achromatic OCT capsule with an optical fiber tether was designed and fabricated.By incorporating a customized micro-diffraction optical element into the optical path,the severe chromatic aberration introduced by the 200 nm broadband spectra was successfully corrected,resulting in an axial resolution of up to 1.8 ?m in air.The capsule-shaped appearance not only facilitates awake patients to take the OCT endoscope into the esophagus without sedation,but also ensures that the endoscope can be placed in the esophagus more steadily,and thus improves imaging stability.The fiber optic tether allows the capsule to be recycled repeatedly after imaging.A capsule OCT endoscope with an optical fiber tether was developed for the tubular structure of the esophagus.The optical design of the probe guarantees an axial resolution of up to 2.6 ?m in air.The mechanical design guarantees that the patient can take the endoscope autonomously like an oral capsule,and the attached tether ensures it can be retrieved for reuse after complete disinfection.Results show that the two endoscopic OCT systems and the attenuation coefficient calculation algorithm developed in this dissertation are capable of performing high-resolution,large-scale and real-time in vivo imaging of the upper gastrointestinal tract,and have great potential in clinical screening and surveillance as well as postoperative re-examination.