Introduction to OCT - OBEL
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Introduction to OCT
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Section 1. Background
Optical coherence tomography (OCT) [1,2] is an imaging technique which works similar to ultrasound, simply using light waves instead of sound waves. By using the time-delay information contained in the light waves which have been reflected from different depths inside a sample, an OCT system can reconstruct a depth-profile of the sample structure. Three-dimensional images can then be created by scanning the light beam laterally across the sample surface. Whilst the lateral resolution is determined by the spot size of the light beam, the depth (or axial) resolution depends primarily on the optical bandwidth of the light source. For this reason, OCT systems may combine high axial resolutions with large depths of field, so their primary applications have included in vivo imaging through thick sections of biological systems, particularly in the human body. The technique has already become established as a standard imaging modality for imaging of the eye, with numerous commercial instruments on the market. The application of OCT imaging to other biomedical areas such as endoscopic imaging of gastro-intestinal and cardiovascular systems is currently an active field of research.
Section 2. Technology
There are two main categories of OCT instrumentation: Time-Domain OCT (TDOCT) and Spectral-Domain OCT (SDOCT). Time-Domain OCT technology is more intuitive to understand, and most early reasearch and commercial instrumentation was based on this technology. Spectral-Domain OCT is rapidly replacing the Time-Domain technology in most applications because it offers significant advantages in sensitivity and imaging speed.
Fig. 1 shows a schematic diagram of the basic fibre-based TDOCT setup. The Michelson interferometer splits the light from the broadband source into two paths, the reference and sample arms. The reference arm is terminated by a mirror which can be scanned in in the sample arm, the light is weakly focused into a sample. The interference signal between the reflected reference wave and the backscattered sample wave is then recorded. The axial optical sectioning ability of the technique is due to the following reason: Because the light is emitted from a broadband source (large range of optical wavelengths), a strong interference signal is only detected when the light from the reference and sample arms has travelled the same optical distance. Specifically, coherent interference is observed only when the optical pathlengths differ by less than the coherence length of the light source, a quantity that is inversely proportional to its optical bandwidth. The act of translating (axially scanning) the reference arm reflector is equivalent to performing optical sectioning of the sample, allowing for the generation of map of optical reflectivity versus depth. Transverse scanning of the sample (to build up a two- or three-dimensional tomographic image) is achieved via rotation of a sample arm galvonometer mirror.
Figure 1. Schematic of a TDOCT system
Section 4. Spectral-Domain OCT (SDOCT)
Fig. 2 shows the basic fibre-based SDOCT setup. Most of the components are identical to the setup of the Time-Domain technology. The key difference is that in an SDOCT system the reference arm length is fixed. Instead of obtaining the depth information of the sample by scanning the reference arm length, the output light of the interferometer is analyzed with a spectrometer (hence the term Spectral-Domain). It can be shown that the measured spectrum of the interferometer output contains the same information as an axial scan of the reference arm. The map of optical reflectivity versus depth is obtained from the interferometer output spectrum via a Fourier Transform. The particular implementation of SDOCT shown in Fig. 2 is also commonly referred to as Frequency-Domain OCT (FDOCT). Another variant of SDOCT uses a wavelength-tunable laser to rapidly sweep through a range of wavelengths, allowing the spectrum of the interferometer output to be recorded sequentially using a single detector. This technique is called Swept-Source OCT (SSOCT) and it is particularly interesting for OCT systems operating at wavelengths longer than 1 micrometer, where expensive InGaAs image sensors would be required for FDOCT.
Figure 2. Schematic of a SDOCT system
Section 5. Comparison of OCT with other imaging modalities
OCT may be directly compared with alternative techniques in terms of several different criteria: resolution, imaging depth, acquisition time, complexity, and sample intrusiveness. With regard to the first two, OCT occupies a niche represented in Fig. 3: its imaging depth is typically limited to a few millimetres, less than ultrasound, magnetic resonance imaging (MRI), or X-ray computed tomography (CT), but its resolution is greater. This comparison is reversed with respect to confocal microscopy. Like ultrasound, the acquisition time of OCT is short enough to support tomographic imaging at video rates, making it much more tolerant to subject motion than either CT or MRI. It does not require physical contact with the sample, and may be used in air-filled hollow organs (unlike ultrasound). OCT uses non-ionising radiation at biologically safe levels, allowing for long exposure times, and its level of complexity is closer to ultrasound than to CT or MRI, allowing for the realisation of low-cost portable scanners. The point-scanning nature of OCT technology allows it to be implemented in fibre optics, which makes endoscopic and catheter-based imaging possible.
Figure 3. Comparison of OCT resolution and imaging depths to those of al the “pendulum” length represents imaging depth, and the “sphere” size represents resolution
Section 6. Recent developments
Recent developments in the OCT field [3] have included the deployment of sources of extremely broad bandwidth, primarily based on supercontinuum generation in various forms of optical fibre, but also employing incandescent sources, to achieve axial resolutions of about 1 micrometer. (Standard current-generation semiconductor sources typically achieve resolutions of about 10 micrometers.) The advantages of polarisation-sensitive imagin sample birefringence (dependence of the phase velocity on polarisation) and dichroism (dependence of the amplitude on polarisation) are being exploited as contrast mechanisms.
OCT research in OBEL is broad, encompassing theoretical modelling, fundamental-level experimental research, system design and implementation, software development, and clinical measurements.
Section 8. References
D. D. Sampson, T. R. Hillman, Optical coherence tomography, Lasers and Current Optical Techniques in Biology, G. Palumbo and R. Pratesi, eds. (ESP
Comprehensive Series in Photosciences, Cambridge, UK, 2004), pp. 481-571.
D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, Optical coherence tomography, Science, 254,
D. D. Sampson, Trends and prospects for optical coherence tomography, in 2nd European Workshop on Optical Fiber Sensors, edited by J. M. Lopez-Higuera, B.
Culshaw, Proc. of SPIE, Vol. 5502, (SPIE, Bellingham, WA, 2004), pp. 51-58.
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OCT三维重建与多普勒成像探究.pdf 66页
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硕士学位论文
OCT三维重建与多普勒成像研究
姓名：吴凌
申请学位级别：硕士
专业：光学工程
指导教师：丁志华
浙江大学硕士论文
光学相干层析成像(Optical Coherence Tomography，OCT)技术是一种具有微
米量级高分辨率的非侵入性生物医学成像手段，无需进行生理切片就能够获得活
体样品内部的三维结构等信息。对OCT所得到的三维信息作可视化处理，可以
有效地帮助研究人员更好的理解所拥有的数据。目前OCT三维重建的有着多种
方法，可以通过不同的表现方式来展现样品内部信息，包括：多平面重组法、投
影法、面绘制、体绘制等。
OCT三维重建的数据来源于从数字信号重组得到的图像。OCT不仅可以获
得样品结构图像，还可以通过多普勒图像重建来得到样品内部的血液流速等重要
信息。为了获取高质量的图像数据，需要对信号进行预处理，并选择合适的图像
重建算法。比如，多普勒图像重建的算法就需要根据不同实验环境的要求来选择
使用短时傅里叶变换法或者希尔伯特变换法。
本课题主要研究了三维重建的各种方法，利用已有的光纤型OCT系统，使
用Visual c++编写三维扫描软件获取三维重建所需要的数据，并使用MATLAB
实现结构图像的三维重建，同时研究了不同算法对多普勒图像重建的影响，并在
小白鼠活体实验中予以验证。
本文正文部分共分为五章。第一章简述了OCT的相关原理，阐述了三维重
建的研究现状与意义，概述了课题中软件开发工具的选择。第二章介绍了OCT
的三维信号的获取，以及模拟信号处理和数字信号处理的相关内容。第三章阐述
了三维重建的各种方法，以及三维重建前对OCT图像预处理的方法，并对三维
扫描实验中所得到的数据进行三维重建和动画渲染。第四章介绍了多普勒图像重
建的两种算法，通过实验比较了两种算法的适用环境。第五章对课题作出总结，
并提出了今后需要努力的方向。
关键词：光学相干层析，三维重建，图像处理，多普勒图像重建，短时傅里叶变
换，希尔伯特变换
浙江大学硕士论文
Optical coherence tomography(OCT)is a noninvasive biomedical imaging method
witll micron scales resolution which could pi'ovide three-dimensional structural
information of tissue in vivo without the need for excision or preparing sections．
Visualization of the three-dimensional OCT information call effectively help
researchers have a better understanding of the data．There are many methods of
OCT three-dimensional reconstruction,providing different means of expression to
show the internal information in samples,including：multi-planar reconstruction，
projection，surface rendering，volume rendering,etc．
11le images reconstructed fi'om OCT signals form the soure嚣of the
three-dimensional reconsauction．The images include no only the structural images
of sample，but also the Doppler images with important information such as internal
blood flow．In order to providing a high quality image data,it is necessary to do
signal preconditioning and choose a suitable image reconstruction algorithm．For
example,the Doppler reconstruction needs to choose the short time Fourier
transformation or the Hilbert transformation，depending on
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