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时域扩散荧光层析技术基本原理与系统研究
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摘要
近红外扩散荧光层析成像技术(Fluorescence diffuse optical tomography, FDOT)作为一种最有前景的小动物成像手段渐渐突显出来。其目的是在特异性荧光探针的指引下,通过测量规则组织边界溢出的荧光信号,对生物体内部生理、病理过程在细胞或分子水平上实现在体动态观察。扩散层析成像技术能够对于目标区域进行全三维定位,并对其所聚集荧光探针的浓度进行量化的分析。时域技术的潜在优势在于通过解析激光脉冲激励下产生的荧光信号而直接获取荧光寿命信息,且能够同时重建荧光产率和寿命以及进行多组分分析。
     本文提出了对于常用的无限平板透射模式及圆域层析模式的时域FDOT的成像重建算法,并且对算法进行了详细的描述。通过数值模拟验证对于算法的空间分辨率及噪声鲁棒性等方面进行了准确的评估。又设计相关实验系统,进一步通过实验验证结果证明算法的可行性。本文提出的时域FDOT算法是利用基于耦合扩散方程拉氏变换的广义脉冲谱技术和基于玻恩比的归一化逆问题模型,从而实现无限平板透射及圆柱层析模式下的图像重建。该方法采用外推边界条件下无限平板和二维圆域结构的扩散方程解析解。为了解决线性求逆过程中的病态问题,采用了代数重建技术进行相应的线性求逆。并通过引入一对实数拉氏变换因子,可实现荧光产率和寿命的同时重建
     尽管基于CCD相机的连续FDOT测量系统已被广泛接收,但光纤式系统具有的超高灵敏度和皮秒级别的时间分辨能力,使其在时域FDOT领域备受关注。如时间相关单光子计数(TCSPC)系统已经成功地在时域扩散光层析系统中得到了广泛地应用。本文针对提出的多维TCSPC系统,采用了基于时间分辨反透射测量技术的混浊介质光学参数重构方法,准确地测量了固态仿体和液态仿体的光学参数。
     本文利用多通道的时间相关单光子计数系统对于透射模式及圆域层析模式的时域FDOT重建算法的可靠性及可行性进行实验验证。其中所用平板型及圆柱型实验仿体中的荧光靶向区域由荧光试剂及1%的类脂肪乳的混合溶液构成。实验中采用了已被广泛的应用于生物学和生物医学研究中的生物组织荧光标记物-Cy5.5和吲哚菁绿。实验结果表明提出的重建算法对于目标体的位置和形状都进行了较准确的重建。但为了提高时域FDOT技术的成像效果,在系统优化,算法开发方及实验量化等方面仍存在大量的工作需要完成。
Near-infrared Fluorescent Diffuse Optical Tomography (FDOT) is emerging as a promising tool for small animal imaging. The modality, with the aid of specific fluorescent probes, aims at in vivo visualizing interior cellular and molecular events from fluorescence signals measured on boundary of an intact tissue. FDOT enables both three-dimensional localization of the targeted areas and quantization of the local concentration of the fluorochromes. The time domain technique offers the potential advantages of directly extracting the lifetime information through temporally resolving the fluorescence responses to a pulsed excitation, and has additionally the favorite performances of simultaneously recovering fluorescent yield and lifetime distributions, as well as resolving multiple components.
     This thesis presents the methodology of time-domain FDOT for slab and circular geometries that are commonly used in practice, involving image reconstruction algorithm, numerical simulations, instrumentation of a multi-channel time-resolved imaging system, and experimental validations. The algorithm is based on a linear generalized pulse spectrum technique that employs the analytical solution to the Laplace-transformed time-domain photon diffusion equation to construct a Born normalized inverse model, which can overcome the impact of the instrumental response function and therefore eliminate the requirement for calibrating the time-origins and the coupling factors of the system. The resultant linear inversions are solved using an algebraic reconstruction technique. A pair of real domain transform-factors is introduced to separate the fluorescent yield and lifetime images.
     Despite the wide adoption of a CCD camera to continuous-wave FDOT, a fiber-based instrumentation is attractive in time-domain FDOT regime since it can make full use of well-established ultra-high sensitive and ps time-resolved detection techniques, such as a time-correlated single photon counting (TCSPC) technique that have been widely used in time-domain diffuse optical tomography with a great success. Based on the proposed multi-channel TCSPC system, a method for determining the optical properties of turbid medium is developed using time-resolved reflection and transmission measurements, and validated on solid and liquid phantoms.
     The feasibility and reliability of the time-domain FDOT methodology are experimentally validated with the multi-channel TCSPC system, on slab and cylinder phantoms, each embedding one or two fluorescent targets made from mixture of 1%-Intralipid solution and Cy5.5 or Indocyanine green (ICG) agent, the two fluorescent agents widely used in biological and biomedical studies. The results show that the approach retrieves the positions and shapes of the targets with a reasonable accuracy. Nevertheless, an investigation must be made in depth on system optimization of the measurement system, modification of the image reconstruction methodology and assessment of FDOT quantification.
引文
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