摘要
作为人体髋关节的重要组成部分,股骨头发生病变甚至坏死的机率很高,目前这仍然是医学领域尚未解决的疑难问题之一。股骨头软骨破坏受损是股骨头发生病变的重要早期表现之一。因此,及时准确地评价关节软骨对于病情的诊断和治疗方法的确定起着非常重要的作用。核磁共振成像(Magnetic Resonance Imaging, MRI)技术以其无电离辐射损害、对软骨组织敏感等优点,成为目前测量和评价关节软骨的最佳无创性方法。利用计算机分析和处理MR图像,实现对软骨厚度的测量和评价,已经逐渐成为医学图形处理领域的一个研究热点。对这项技术的研究将会大大提高诊断的效率和治疗的效果,也符合医学图像处理的发展趋势。本文就是以MR图像中股骨头软骨的图像分割、厚度测量、三维重建等几项关键的检测技术为主要研究内容的。
在图像分割方面,结合了不同分割方法各自的优点,提出了融合多种信息的多步分割算法,实现了对MR图像中股骨头软骨的快速自动分割。针对经过预处理的MR图像,通过Hough变换计算出股骨头的圆心位置,结合股骨头的解剖学尺寸约束,提取出图像中的感兴趣区域,并实现粗分割。利用自适应阈值的Canny边缘检测算子提取目标区域的边缘,按照提出的准则对粗分割区域内的边缘进行噪声消除,检测出精确的股骨头软骨内外边缘,进而提取出内外边缘之间的图像信息,实现对软骨的准确分割。
在厚度测量方面,在MR成像平面内实现对软骨厚度的测量是目前的主要技术手段。通过建立平面薄面体的数学模型和MR摄影过程的仿真信号,在理论上证明了基于高斯二阶微分的零交叉法对于二维MR断层图像中目标特征厚度测量的有效性,仿真实验和MR成像实验进一步验证了这个结论的正确性。在此基础上,针对由软骨表面形状和断层成像引起的软骨厚度的过测量问题,提出了基于形状约束的过测量误差校正算法,对断层图像中得到的测量厚度进行校正,准确地获得了软骨的真实厚度信息,实现了在二维MR断层图像中股骨头软骨的准确测量。
在三维MR图像中直接测量股骨头软骨的空间厚度将是测量技术的发展趋势,因为这种方法无需校正环节,在原理上更符合测量的物理意义。由于MR图像断层间的距离远大于图像中像素间的距离,因此对断层图像层间进行插值是实现空间测量的必要前提。提出了基于灰度区域分割的线性插值与匹配插值混合的插值算法,将线性插值的高效性和匹配插值的精确性有机结合,较好地实现了断层图像间的快速准确插值,使空间体素具有各向同性特征。在此基础上,将基于高斯二阶微分的零交叉法拓展到三维空间,用Hessian矩阵简化了复杂的高斯函数三维卷积运算,实现了对软骨空间厚度的直接测量。实验表明这是一种有效的测量方法。
在三维重建方面,利用经典的三角剖分方法实现了股骨头软骨的表面重建。针对由密集的量化数据点带来的表面粗糙问题,提出了层间错位的数据点遴选方法,成功地消除了重建表面上的梯田效应。针对空间直接剖分产生的耗时问题,提出了改进的Delaunay三角剖分方法,在保证了重建表面质量的前提下,有效地减少了计算时间,提高了重建效率,增强了三维重建方法的应用性。重建的软骨三维模型中包含了软骨的形状特征和厚度信息,对于医学诊断、康复监测以及软骨置换术等治疗方法具有重要的参考价值。
As one of important parts in human hip joint, the femoral head has high probability to get lesion or even to become necrotic. This is still one of the knotty problems in medical field at present. The damage on the femoral cartilage is one of the major features in early stage of diseases on the femoral head. Thus, it is very important to evaluate the articular cartilage timely and accurately in order to make a diagnosis and to determine the therapeutic method. With the advantage of no damage of ionizing radiation, and being sensitive to the cartilage tissue, the technology of Magnetic Resonance Imaging is regarded as the best noninvasive method of cartilage detection and evaluation at present. Using the computer to analyze and process the MR images, and to estimate and evaluate the thickness of cartilage, has been gradually considered as a research hotspot of medical image processing. Research on this technology will greatly improve the efficiency of diagnosis and the effect of treatment, which is also coincident with the development trend of medical image processing. In this thesis, the main research content is several key detection techniques in MR images for femoral cartilage, including image segmentation, thickness measurement, 3D reconstruction.
On the aspects of image segmentation, combining respective advantages of various methods, a multi-step segmentation based on information fusion was proposed to implement fast automatic segmentation of the femoral cartilage in MR images. After pretreatment, the MR images were used to calculate the center of the femoral head by imposing the Hough transform. Combining the anatomic constrain of the femoral head, the region of interest (ROI) was selected and the rough segmentation was realized. The image edges of the object region were then extracted using the adaptive thresholding Canny detector. According to the properties of the pixel on femoral cartilage edge, we labeled these edges and removed the noise edges according to the custom rules to acquire the exact edges of the femoral cartilage. Finally, the femoral cartilage was segmented by extracting the image information between the cartilage edges.
On the aspects of thickness measurement, the main technique at the present time is to measure the thickness of cartilage in the MR image plane. By building mathematical modal of 2D sheet structure and simulating signals in the process of the MR photographing, it was proved theoretically that the zero-crossing method based on the second directional derivatives of Gaussian blurring was effective to measure the sheet structure thickness in 2D MR images. The simulation experiments and MR photographing experiments further verified the conclusion. Based on it, aiming at the problem of overestimation due to the surface shape of cartilage and the slice imaging, a correcting algorithm based on shape constraint was proposed to correct the in-plane thickness, and the real thickness value of femoral cartilage in the 2D MR images was obtained accurately.
Measuring the cartilage thickness of femoral head directly from the 3D MR images will be the trend in the development of measuring technology. Without need of correction, this method is more significance physically in the principle of measurement. The distance between neighbor slices in MR images is much bigger than that between the neighbor pixels in the slice image, so it’s necessary to interpolate between the image slices in order to performance the spatial measurement. A mixed interpolation algorithm based on segmentation of gray region was proposed, which combined the efficiency of linear interpolation and the accuracy of matching interpolation organically, and achieved a good interpolation of the images among the slices to make the voxel isotropic. On the basis of this, the zero-crossing method based on the second directional derivatives of Gaussian blurring was extended to 3D space. The complex 3D convolution of Gaussian function was simplified by Hessian matrix to realize the measuring the spatial thickness of cartilage directly. Experiments showed that this is the affective method of measurement.
On the aspects of 3D reconstruction, the classical triangulation methods were used to reconstruct the surfaces of the femoral head cartilage. Aiming at the problems of surface roughening caused by the dense quantitative points data set, a data points selection method based on cross dislocation was proposed to eliminate the terrace effect on the reconstruct surface successfully. To solve the problem of time consuming during the spatial direct triangulation, an improving Delaunay triangulation method was proposed, which not only guaranteed the quality of the surface reconstruction, reduced the calculating time effectively, but also improved the reconstruction efficiency, strengthened the applicability of the 3D reconstruction method. The reconstructed 3D model of the femoral head cartilage included both the shape features and the thickness information of the cartilage, which could possess important reference value to medical diagnosis, rehabilitation monitoring and many treatment methods, such as cartilage replacement.
引文
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