Development and Application of Polarization-Sensitive Optical Coherence Tomography

Abstract

Optical coherence tomography (OCT) is an interferometric technique capable of non-invasive high-resolution cross-sectional imaging, and it has been widely used in biomedical imaging ever since its introduction. Polarization-sensitive OCT (PS-OCT) is a functional extension of OCT having polarization-sensitive contrast. This additional contrast is useful for distinguishing lesions from normal tissues or detecting tissues having polarization properties. In a past decade, PS-OCT has been advanced in both the imaging system and data analysis method, however strong specular reflection of samples has still been a problem in implementation of PS-OCT for clinical setting. Because this strong specular reflection can induce image quality degradation due to signal saturation, and make the polarization measurement difficult. Recently a dark-field method which separates the illumination and detection light paths was applied to OCT in order to avoid the specular reflection problem, and it may be beneficial in the practical applications of PS-OCT as well. In this study, dark-field PS-OCT which can avoid the specular reflection problem is presented. It was implemented by adapting the dark-field method using Bessel-beam illumination and Gaussian-beam detection, and a PS-OCT method based on passive delay unit (PDU). The new system was characterized in comparison with the conventional Gaussian-beam based method in both polarization components and various samples including the human skin. Dark-field PS-OCT performed as good as the conventional PSOCT without the specular reflection artifact. Dark-field PS-OCT may beuseful in practical situations where the specular reflection is unavoidable. Another demand of implementation of PS-OCT in the clinical setting is endoscopic imaging probe for assessing internal organs, due to the inherent imitation of OCT imaging depth. Also, combination PS-OCT with other imaging techniques can be advantageous in providing additional information. In this study, a forward-viewing multimodal imaging probe combining wide-field reflectance/fluorescence and PS-OCT is presented. This multimodal imaging probe was applied to in-vivo human oral cavity tissues, and information of surface morphology and vasculature, and under-surface layered structure and birefringence of the oral cavity tissues was simultaneously obtained. It will be useful for studying internal organs such as oral cavity lesions in clinical application. Finally, a biomedical application of PS-OCT, visualization prostatic nerves by PS-OCT are presented. Preservation of prostatic nerves is critical to recovery of a man’s sexual potency after radical prostatectomy. A real-time imaging method of prostatic nerves will be helpful for nerve-sparing radical prostatectomy (NSRP). PS-OCT provides a distinctive contrast of nerve by its polarization property, therefore detection of the prostatic nerves by PS-OCT is possible. In this study, PS-OCT was applied for detection of prostatic nerves in both ex-vivo rat prostates and human prostate specimens, and its feasibility was demonstrated. In conclusion, this dissertation presents developments of advanced PS-OCT and its biomedical applications. A novel dark-field PS-OCT system which can avoid specular reflection problem, and a forward-viewing multimodal imaging probe combining wide-field reflectance/fluorescence and PS-OCT which enables to assess internal organs are presented. In addition, visualization of prostatic nerves by PS-OCT is presented as the biomedical application. These results will contribute the new perspectives in practical implementations of PS-OCT as well as a basis for intraoperative use of PS-OCT.