【作者】 李琦；

【导师】 姬红兵；

【作者基本信息】 西安电子科技大学 ， 模式识别与智能系统， 2013， 博士

【摘要】 多模态医学影像根据所提供的信息不同可分为两大类：解剖影像和功能影像。解剖影像提供较高分辨率的组织或器官的解剖结构信息，但无法反映其代谢情况；功能影像反映人体组织或器官的功能和代谢信息，但分辨率较低。为了将这些互补信息有机地融合在一起，先决条件就是进行多模态医学影像的配准，配准的精度直接决定着融合的效果。医学影像配准就是寻求一种最优空间变换，使两幅医学影像的对应点在给定的相似性测度下达到空间位置和解剖位置的一致。在不同模态医学影像成像的过程中，通常将用于成像的放射线、示踪剂和强磁场等对人体有害的影响控制在一定的范围内，加上成像模式本身的一些物理限制，往往导致影像不清晰，并伴有噪声甚至伪影的出现。因此，研究多模态医学影像配准方法对噪声的鲁棒性具有十分重要的理论意义和应用价值。本文取得的主要创新研究成果如下：(1).针对传统互信息测度的局限性，提出了一种基于主定序和混合熵的多模态医学影像鲁棒配准方法。首先利用主成分分析(PCA)方法定义了医学影像的主定序特征，描述影像邻域像素间的空间信息和微观结构特性；进而结合影像灰度构建了一种基于混合熵的配准测度，有效保证了配准测度函数的光滑性和收敛性。多组多模态医学影像测试结果表明，提出的方法能有效抑制噪声，具有很高的配准精度，鲁棒性强，优于现有的几种方法。(2).针对非线性高维定序特征，提出了一种基于局部线性嵌入(LLE)和混合熵的多模态医学影像鲁棒配准方法。该方法采用LLE算法对医学影像多方向的定序特征进行降维处理，并通过LLE逆向映射方法重构医学影像的整体定序特征，最后结合影像灰度构成特征空间，基于混合熵的配准测度通过衡量两幅影像对应灰度信息和空间信息的一致性，使配准测度函数更加平滑。实验结果表明，在影像空间分辨率较低和有噪声的情况下，该方法具有精度高、鲁棒性强的特点。(3).针对医学影像的非刚性配准，提出了一种基于自由变形(FFD)和改进的L-BFGS优化的配准新方法。首先运用仿射变换对待配准影像进行全局粗配准，再对发生局部形变区域采用基于B样条的FFD模型进行非线性空间变换，并将基于LEE和混合熵的配准测度应用于非刚性配准中，在最优配准参数的优化搜索过程中，改进的L-BFGS优化方法采用限制步长和变异操作增强了跳出局部最优解的能力，提高了搜索精度。实验结果表明，该方法在运算速度和配准精度上有较明显的优势，但不适用于噪声情况下影像中存在较大组织形变的配准。(4).针对配准的物理模型，提出了一种基于局部保留投影(LPP)和粒子群优化(PSO)的医学影像鲁棒配准方法。在医学影像配准过程中，将浮动影像向参考影像配准所产生的一系列空间变换看作视频流中的连续帧，则医学影像配准过程可以看作是一个光滑而连续的视频流，从而配准空间存在于某种低维非线性流形上，但该流形的结构非常复杂，很难与众多形变参数的物理意义对应起来。该方法采用流形学习的LPP算法配合PSO算法进行配准参数的优化搜索，避免了对配准流形直接建模的困难。由于数据降维过程中大量信息被约简，该方法的配准精度要略低于本文之前提出的算法，但其优势在于适用性好，不仅适用于医学影像的刚性、非刚性配准，而且可以推广到其他各种场合的图像配准。更多还原

【Abstract】 According to the physical principles for imaging, multimodality medical imagesare usually divided into two types: structural and functional images. Structural imagesprovide mainly high-resolution images with geometric and anatomical information.Functional images provide metabolic or neurochemical changes characteristic but withcoarser resolution. In order to fuse complementary characteristics of the underlyinganatomy and tissue microstructure from medical images of different modalities, thegeometric registration is a preliminary and crucial step and has a paramount influenceon the performance of image fusion. Medical image registration is defined as theprocess that determines the best structural and physiological correspondence betweentwo or more medical images.In multimodality medical imaging, medical images are sometimes blurringbecause the adverse influence of the radioactive ray, tracer and high magnetic field forhuman body must be reduced. Moreover, there are the physical limitations of theimaging process itself, such as noise, limited resolution, insufficient contrast orinhomogeneity. Therefore, robust registration of multimodality medical images is avery challenging problem. The main achievements of this dissertation are as follows:(1). To improve the robustness and precision of the maximization of mutualinformation (MI) similarity, a principal ordinal feature and hybrid entropy basedregistration method is presented. A principal ordinal feature (OF) is defined and used torepresent the spatial information between the neighboring pixels and the properties ofthe specified micro-structure in medical images. Integrating with pixel intensities, asimilarity measure based on hybrid entropy is defined to register multimodality images.The proposed method is demonstrated using several pairs of multimodality medicalimages and the experimental results show that the noise of images can be effectivelysuppressed and compared with some existing methods, the proposed registrationalgorithm is of higher precision and better robustness.(2). Due to the nonlinear relationships of high-dimensional OFs in nature, a locallinear embedding (LLE) and hybrid entropy based registration method is proposed. Forhigh dimensional OFs, the LLE algorithm is used to dimensionality reduction and theinverse mapping of LLE is used to fuse complementary information of OFs together.Then a novel similarity measure based on hybrid entropy which integrates intensity with OF is defined to register multimodality images. Through quantitative evaluations,the proposed measure is proven to provide improved robustness with accuracy.(3). Non-rigid registration of medical images has become a challenging task inmedical image processing and applications. A LLE and improved L-BFGSoptimization based registration method is proposed. A hierarchical transformationmodel of medical images is developed. The global motion is modeled by an affinetransformation while the local motion is described by a free-form deformation (FFD)based on B-splines. Then the proposed LEE and hybrid entropy based similaritymeasure is chosen as the registration function. Finally an improved L-BFGS algorithmis used to search the optimal registration parameters. We evaluate the effectiveness ofthe proposed approach by applying it to the simulated brain image data. Theexperimental results show that the proposed registration algorithm is of higherprecision and faster speed. However, it cannot be applied to image registration withlarge anatomical variation in noisy environment.(4). Based on a new physical model of image registration, a locality preservingprojection (LPP) and particle swarm optimization (PSO) based registration method isproposed. A large deformation between the reference image and the floating image isdecomposed into a series of small deformations and the result of every deformation istreated as a frame in the registration video. Therefore, the registration process isequivalent to a smooth and continuous video and the registration space is on a certainnonlinear manifold of lower dimensionality. However, the registration manifold isusually too complicated to build the correspondence with numerous deformationparameters. Hence, medical images are projected into manifold space by the LPPalgorithm and the optimal registration parameters are searched by the PSO algorithm.Though the proposed method is not as good in precision as what we expect because ofdimension reduction, it can be widely used to many applications.更多还原