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MRI颅脑图像分割算法研究
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摘要
随着各种影像检查设备在临床上的应用,医学图像的后处理技术成为近年来图像处理领域中的研究热点问题,而图像分割是图像的三维重建、可视化、配准、融合和定量分析等技术的前提和基础。本文在研究常用图像分割算法的基础上,主要探讨图割(Graph Cut)算法在MRI脑组织图像分割中的应用问题,任务是实现脑灰质(Gray Matter, GM)、脑白质(white matter, WM)和脑脊液(cerebrospinal fluid,CSF)等组织的分割。
     对于最小割的求解问题,本文采用“预流”(preflow)算法求网络最大流,获得最小割集,实现图像的分割,提高了算法的运算速度。
     针对归一割算法计算速度慢的弱点,本文将分水岭变换引入到图割(GraphCut)框架中,提出一种新的基于分水岭预分割的快速归一割(sNcut)算法。图割算法应用到图像分割中,其核心是根据某个模型将图像映射成一个加权无向图,并寻求图的最优割集,从而实现图像目标区域与背景区域的分割。将分水岭变换分割出来的若干个内部像素密切相关的小区域映射成图的顶点,可大大减少顶点的数目,运算速度较像素点的直接映射方法明显提高。
     针对分水岭算法本身的过分割问题,以及噪声信号对过分割的助长现象,本文设计了一个多结构的自适应形态滤波器,消除图像中的噪声,并在分水岭分割之后采用添加标记点法进行区域合并,这种分割的前处理和后处理有效地抑制了过分割现象,保证预分割的结果是有意义的,为图的构建及最优割集的寻找提供前提条件。
     另外,为了保证归一割算法分割的准确性,本文对权重的确定方法进行了改进,一般的方法是利用像素的灰度梯度确定权重,建立图模型,本文综合利用像素的灰度和空间两方面的信息确定顶点之间的权重,提出一个新的权重模型ISI(indensity and spatial information, ISI)。
     为了验证算法的有效性,本文采用40套脑图像数据进行仿真实验,其中35套数据的分割结果达到预期目的,可以准确地将脑灰质、脑白质和脑脊液等组织分割出来,其它5套数据由于图像质量导致无法分割或分割错误,其分割的正确率为87.5%,该实验结果表明本文提出的算法是有效的。
With the clinical application of all kinds of imaging equipments, the medical image post-processing technology has become the hot issue in the recent image segmentation field, while the image segmentation is the premise and foundation of the three-dimensional reconstruction, visualization, registration, integration and quantitative analysis techniques. Based on the research of several usual image segmentation algorithm, the paper focuses on the use of Graph Cut algorithm in the MRI brain image. And the task is to achieve the partition of brain gray matter (GM), white matter (WM) and cerebrospinal fluid (CSF).
     For the minimum cut solution of the problem, this paper adopts the "preflow" algorithm for the maximum flow of the graph network in order to attain the minimum cut set. The method achieves the image segmentation algorithm and improves computational speed.
     Owing to the normalized cut (Ncut) algorithm's weakness of calculating slowly, the paper uses the watershed transformation algorithm in the graph cut framework and propose a new speedy normalized cut algorithm—sNcut algorithm. The graph cut is applied to the image segmentation, whose core is to map an image into a weighted undirected graph based on a certain model and then search for the optimal cut set in order to achieve the partition of the object region and background region. A number of small regions, split by the watershed transformation, whose internal pixels are closely related to one another, are mapped the vertices in the graph. This method can significantly reduce the number of vertices and the computing speed has improved significantly than the direct pixel mapping method.
     Direct at the over-segmentation problem in the watershed processing and the phenomenon that noise signal encourages the over-segmentation, the paper designs a multi-structure adaptive morphological filters to remove the image noise and proceeds the region combination by adding marker points. The pre-processing and post-processing of segmentation effectively inhibits the phenomenon of over-segmentation to ensure that the result of the pre-segmentation is meaningful and to provide a prerequisite for the graph construction and searching for the optimal cut set.
     In addition, in order to ensure the accuracy of the normalized cut segmentation, the paper improves the method of determining weights. The general approach is to use the pixels'gray gradient to determine the weight and establish the graph model. The paper combines the pixels'grayscale and space information to determine the weight between the vertices and proposes a new model of weight--indensity and spatial information (ISI).
     In order to verify the effectiveness of the algorithm, this paper uses 40 sets of brain image data for simulation experiments. However the segmentation results of 35 sets of data achieves the intended purpose and cerebral gray matter, white matter and cerebrospinal fluid and other organizations can be partitioned accurately. The other five sets of the data can not be divided or split error due to image quality. The correct rate of segmentation is87.5%, and the experimental results show that the proposed algorithm is effective.
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