【作者】 吴彤；

【导师】 丁志华；

【作者基本信息】 浙江大学 ， 光学工程， 2011， 博士

【摘要】 光学相干层析成像(Optical Coherence tomography, OCT)技术是一种无损伤、非侵入、高分辨率的成像技术,能够对生物组织内部结构和生理功能进行在体成像。扫频OCT (Swept source OCT, SS-OCT)属于傅立叶域OCT技术,与时域OCT技术相比,在系统的成像速度和灵敏度方面都有显著的提高,已成为目前OCT领域的研究热点,在生物组织成像、内窥成像等领域发挥了重要作用。本课题对扫频OCT成像方法及系统技术进行了研究,具体的研究内容及获得的创新性研究成果有：1.建立了由扫频光源、主干涉仪和标定干涉仪组成的1300nm波段扫频OCT系统,编写了成像系统软件。搭建的扫频OCT系统轴向扫描速率由原来时域OCT系统的500Hz提高到20KHz。系统理论轴向分辨率为7.2μm,最大成像深度为8mm,最大信噪比为110dB,成功获得了人体皮肤、舌头、鱼眼以及矿石的在体实时层析成像。2.研制了基于扫频OCT成像系统的正弦驱动谐振光纤悬臂一维扫描探头。该探头利用光纤悬臂的共振特性,通过对压电陶瓷施加接近于该共振频率的正弦驱动信号,实现光纤悬臂的一维谐振扫描。应用所研制的光纤扫描探头于搭建的扫频OCT系统,在20KHz的A-Scan速率下,成像速度达20帧／秒,横向范围1毫米,横向分辨率10μm,轴向分辨率达到8.3μm,获得了多层样品的层析图像,为内窥探头的研制奠定了基础。3.提出并研制了单驱动正交谐振非对称光纤悬臂二维扫描探头,用于样品横断面的扫描成像。该探头利用光纤悬臂的共振特性,结合光纤悬臂的非对称结构,使光纤悬臂在正交方向具有不同的共振频率。通过对压电双晶片施加接近于上述两种共振频率的混频信号,就能同时激发光纤悬臂在两正交方向上的振动,实现非对称光纤悬臂的二维扫描。探头中的振动驱动件采用压电双晶片取代压电陶瓷管,压变系数大、驱动电压低,提高了应用安全性。混频驱动信号中双频成分幅度与频率的调整,可形成不同扫描范围和不同填充率的李萨如图形扫描轨迹。为了实现扫描图像的正确重建,在扫描探头中我们引入了二维位置敏感探测器,用于扫描轨迹的同步记录。应用所研制的探头于建立的OCT系统,获得了典型样品的横断面图像,达到了预期效果。4.为了保证系统轴向分辨率以及信噪比,需要对干涉光谱信号进行波数(k)空间均匀化标定。基于搭建的扫频OCT系统,提出并实施了基于扫频OCT主干涉仪的预先标定方法、实施了基于马赫曾德干涉仪(MZI)强度信息的实时标定方法、提出并实施了无需光谱标定直接通过非均匀傅立叶变换重建图像的方法,提出并实施了基于MZI相位信息进行k空间直接插值的方法。并且比较了提出的方法与现有标定方法在性能上的优势,在获得波数空间均匀分布的扫频OCT成像信号的同时,减少了计算时间,重建了高质量的生物组织层析图像。5.首次提出了基于透射式光栅型光学延迟线的全量程成像扫频OCT系统的方法,在扫频OCT系统的参考臂中引入透射式光栅型光学延迟线,基于光学延迟线中反射镜与光轴垂轴面的特定夹角,在各光谱成分中引入随光谱波数线性变化的附加位相量,实现参考光的特定群延迟。群延迟后的参考光与样品光汇合,实现干涉光谱的波数载频。对干涉光谱实施基于快速傅立叶变换的图像重建,其共轭镜像将偏离无载频时的位置,由于此偏移量远大于在各光谱成分中引入的附加光程量,确保了在干涉信号灵敏度下降最小化的前提下,有效分离直流项与共轭镜像,实现扫频光学相干层析的全量程成像。对活鱼眼前节成像的结果表明,引入k空间载频的方法实现了扫频OCT的全范围成像,复共轭抑制率高达50dB。更多还原

【Abstract】 Optical, coherence tomography (OCT) is a non-destructive, non-invasive, high-resolution biomedical imaging technique, which can be used to obtain the structure and functional images of the biological internal tissues. Swept source OCT is a subtype of Fourier domain OCT, and compared with Time domain OCT it has the advantages of higher imaging rate and sensitivity. Swept source OCT has become a hot topic of the biomedical imaging research, and play an important role in the field of biological tissue imaging and endoscopic imaging. This dissertation focuses on the imaging method and system technology of SS-OCT, and the main works and innovations are listed as below:1. The SS-OCT system working at 1300nm is developed with imaging software based on VC++. Based on a high speed scanning laser source, the constructed SS-OCT system realizes high speed A-Scan rate of 20 KHz compared to 500Hz A-Scan rate of Time-Domain OCT. The SS-OCT system is mainly comprised of a swept laser source, a primary interferometer and a calibration interferometer. The SSOCT system realizes optical coherence tomographic imaging at axial scanning speed of 20 KHz with axial resolution of 7.2μm and depth range of 8 mm in air. We get some high speed optical coherence tomography images of finger-pad organism, togue tissue, fish eye and mineral stone using the SS-OCT system successfully.2. A sinusoidal waveform driven resonant fiber cantilever based one dimensional scanning probe is proposed and developed based on the constructed SS-OCT system. The probe exploits resonance of a fiber cantilever which is excited by one piece of piezoelectric bimorph through a sinusoidal driving signal whose frequency is near the resonant frequency of the fiber cantilever. The system axial resolution achieves 8.3μm. Based on the fiber cantilever probe, at 20 KHz A-Scan rate, the SSOCT system can get OCT images of sample at a frame rate of 20fps, the lateral range of lmm, and the lateral resolution of 10μm. Some preliminary images of mutilayered sample and biological sample are obtained and presented demonstrating the feasibility of the scheme and settle a steady frame for endoscopic research.3. A fiber based probe that is capable of two-dimensional scanning applicable in optical coherence tomography is proposed and developed. Based on the resonance of the fiber cantilever with asymmetry structure which has two distinguished resonant frequencies in orthogonal directions, Lissajous pattern is produced suitable for two-dimensional scanning upon a sample. Orthogonal resonances of the fiber cantilever are simultaneously excited by single piezo bender actuator with one driving signal consisting of two components corresponding to above-mentioned two resonant frequencies. By integrating a backward-placed two-dimensional position sensitive detector (PSD) into the probe, real-time lateral position of the scanning pattern is registered simultaneously for image reconstruction. Dynamical characteristics of the fiber cantilever are experimentally studied with special consideration on factors determining the resolution of the scanning pattern, including frequency and amplitude ratios between two components of the driving signal and fetching duration used for an cn face image. With the developed probe implemented in the established OCT system, en face OCT images of typical samples are obtained with satisfying resolution and contrast, demonstrating the feasibility of such fiber cantilever with asymmetry structure for realizing two dimensional scanning by single actuator, potentially applicable to endoscopic OCT imaging.4. To ensure the axial resolution approaching the practical value, the image reconstruction method needs the spectral interference signal is evenly distributed in the wave-number (k) space. Based on the constructed SS-OCT system, a calibration method in advace based on the primary interfereometer is proposed and experimentally tested. Non-uniform Discrete Fourier Transform (NDFT) method is introduced in the SSOCT system for data processing. Spectral calibration method based on Mach-Zender Interferometer (MZI) and conventional data interpolation method is also adopted in the system for comparison. OCT images from SSOCT based on the NDFT method, the MZI method and the interpolation method are illustrated. A direct k-space interpolation based on the spectral phase of MZI calibration signal is proposed and developed. The image quality by different calibration methods is compared and ensured the advantage of the proposed calibration method.5. A high speed complex conjugate mirror image elimination method based on the high speed swept source optical coherence tomography system is proposed and developed. The system introduces group delay modulation into the spectral interference signal by a transmissive grating based optical delay for separating the complex conjugate artifact. The autocorrelation function corresponding to each wave number of the source remains a higher level by tilting a small angle of the mirror in transmissive grating based optical delay compared with simply shifting reference mirror. The method is validated experimentally for in vivo imaging.更多还原