肝脏病变MRI诊断与计算机辅助诊断
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摘要
原发性肝细胞癌(HCC)是恶性程度极高的肝脏原发性肿瘤,而肝硬化再生结节(RN)多进展为HCC,从肝硬化中检出小HCC,使HCC病人的成功手术治疗和治愈成为可能;另外,早期肝硬化(肝纤维化)的准确诊断,使肝纤维化病人有望通过内科治疗而痊愈。近几年,核磁共振成像(MRI)已成为肝硬化、HCC影像诊断最敏感的手段之一。但早期肝硬化组织往往形态学变化轻微,同时临床MRI诊断肝硬化缺乏客观量化指标,使常规MRI诊断早期肝硬化存在困难;虽然新型MRI对比剂超顺磁性氧化铁(SPIO)的成功应用,使RN、HCC结节的鉴别诊断水平得以进一步提高,但SPIO增强图像不能显示肝脏病灶的血流动力学特点,同时SPIO增强图像使肝脏病灶诊断的假阳性率提高,目前,RN、HCC结节的MRI鉴别诊断仍为一难题。
     本文针对临床RN、HCC结节及早期肝硬化MRI诊断中存在的问题,将动物模型、临床影像诊断技术与医学图像处理和分析技术相结合,发挥多学科交叉的优势,进行了肝脏病变的MRI诊断与计算机辅助诊断研究。首先,通过建立大鼠HCC模型获取RN、HCC结节钆喷替酸葡甲胺(Gd-DTPA)与SPIO增强图像,评估了Gd-DTPA与SPIO双对比增强图像鉴别诊断RN、HCC结节的应用价值,进一步设计了一种基于纹理特征的神经网络分类器用于SPIO增强图像RN和HCC结节的分类识别;其次,应用本文分类器分类识别肝硬化及正常肝脏病人的MRI;最后,进一步采用遗传算法优化基于纹理特征的BP网络分类器。
     本文所完成的主要研究工作如下:
     (1)大鼠RN、HCC结节SPIO增强图像及SPIO与Gd-DTPA双对比增强图像分析,提高了RN、HCC结节的影像鉴别诊断能力。首先,通过小剂量二乙基亚硝胺诱癌法建立大鼠肝硬化、HCC模型,获得病理证实的肝脏结节106枚(RN 24, HCC 82)。将106枚肝脏结节的SPIO增强图像及SPIO与Gd-DTPA双对比增强图像与病理结果对照分析,结果显示SPIO增强图像HCC结节的对比噪声比(CNR)显著提高(P<0.05),而肝硬化组织的CNR没有显著改变(P>0.05);SPIO与Gd-DTPA双对比增强图像的HCC结节检出敏感性(96.34%)较单一GD-DTPA增强图像的敏感性(89.02%)有一定的提高,但其统计学差异没有显著性(P>0.05)。上述研究结果表明大鼠RN、HCC结节鉴别中,SPIO增强图像能较平扫图像提高HCC结节的检出和定性诊断能力,SPIO增强图像是Gd-DTPA增强图像有益的补充。
     (2)基于纹理特征的BP神经网络分类器用于大鼠及病人肝脏MRI病灶分类识别,获得了较高的分类识别率。首先,利用ISODATA算法自动分割T1WI大鼠肝脏边缘,比较与Otsu阈值选择法和迭代阈值选择法的分割效果,结果显示ISODATA算法更适合于类别未知组织的自动分割问题,为进一步寻找病灶感兴趣区域(ROI)奠定了基础。其次,手动选取大鼠RN、HCC结节SPIO增强图像和病人肝硬化组织及正常肝脏组织T1WI的ROI,并进行纹理特征提取。综合比较后选择基于灰度共生矩阵的纹理特征提取方案,结合医生经验提取二阶矩、对比度、相关、逆差矩、熵、方差六个类间差异显著的纹理特征。针对大鼠RN、HCC结节SPIO增强图像,进行上述纹理特征的统计学分析,选取两组间差异显著的六个纹理特征,并与医生临床经验对比获得一致结论,设计基于纹理特征的神经网络分类器,获得了91.67%的独立测试正确率;针对正常肝脏和肝硬化病人T1WI,统计学分析后选取两组间差异显著的四个纹理特征,设计基于纹理特征的神经网络分类器,获得了87.60%的独立测试正确识别率。两组结果表明本论文所设计的基于纹理特征地的BP网络分类算法适合于大鼠RN、HCC结节SPIO增强图像及肝硬化病人T1WI分类识别,但肝硬化病人T1WI分类识别略低。
     (3)针对肝硬化病人T1WI分类识别,增加纹理特征并基于遗传算法进行BP网络优化。首先,灰度共生矩阵增加至四个方向提取纹理特征参数,每个方向确定14个纹理特征参数,共计54个特征参数。采用盒状图分析后,获得可分性较好的24个特征参数,从而实现特征选择。在输入特征较多的情况下,本文并未直接进行BP神经网络设计,而采用遗传算法辅助下的BP网络优化设计:通过遗传算法优化BP神经网络初始值的GA-BP算法,在保证分类准确率的前提下,不仅增加了BP网络的稳定性,同时使BP网络的平均收敛速度提高,获得了95.00%的独立测试分类准确率。
Primary hepatocellular carcinoma (HCC) is a malignant primary liver cancer, and HCC usually occurs as a complication of cirrhosis. Small HCC detected from cirrhotic background makes the successful surgical treatment of HCC and cure being possible. Fibrosis is an early stage of cirrhosis and fibrosis is expected to recover through medicine treatment if it is diagnosed accurately. In recent years, Magnetic resonance imaging (MRI) has been one of the most sensitive methods for diagnosis of cirrhosis and HCC. But it is difficult to diagnose of fibrosis by MRI because morphological changes of fibrosis are often mild. Successful application of novel MRI contrast agent-superparamagnetic iron oxide (SPIO) improves the ability of differential diagnosis regenerative nodule (RN) from HCC nodule, but the SPIO-enhanced imaging can not show the hemodynamic characteristics of hepatic lesions and the false positive rate of SPIO-enhanced imaging is high relatively. It is difficult to diagnose RN from HCC nodule by MRI now.
     For the problems of imaging diagnosis of fibrosis and hepatic nodules correlation cirrhosis, this reseach integrates animal expetiments and clinical imaging with imaging processing and imaging analysis technology. Possessing advantages of multiple subject crossing, it carries through the investigation of MR imaging diagnosis and computer aided diagnosis of hepatic lesions. Firstly, this dissertation studies the diagnostic value of SPIO-enhanced imaging and combined SPIO-enhanced and Gadolinium DTPA (Gd-DTPA) enhanced imaging for differential diagnosis of RN and HCC nodule through establishment of rat HCC model. Secondly, a neural network (NN) classifier based on texture feature is designed to distinguish rat hepatic nodule in SPIO-enhanced imaging. Thirdly, clinical patient cirrhosis MR imaging is classified by the NN classifier based on texture feature. Lastly, the initial value of the NN classifier for clinical patient cirrhosis MR imaging is optimized by Ggenetic algorithm (GA) to improve the performance of the classifier. The main research work and contributions of this dissertation can be summarized as follows:
     (1) Analysing rat RN and HCC nodule SPIO-enhanced imaging and combined SPIO and Gd-DTPA enhanced imaging and the ability of rat RN and HCC differentiated diagnosis is improved. Rat HCC model is induced by low dose diethylnitrosamine, a total of 106 nodules including 24 RN and 82 HCC are pathologically confirmed. SPIO-enhanced imaging and combined SPIO and Gd-DTPA enhanced imaging of rat 106 modules are analysized compared with pathology. The results show SPIO-enhanced imaging contrast noise ratio (CNR) of HCC nodule is higher than the pre-SPIO enhanced imaging (P< 0.05), SPIO-enhanced imaging CNR of cirrhosis tissue has not significantly changed (P>0.05). The sensitivity of the combined SPIO and Gd-DTPA imaging (96.34%) is higher than the sensitivity of Gd-DTPA imaging alone (89.02%), but the statistical difference is not significant (P>0.05). It is concluded that SPIO-enhanced imaging can improve rat HCC nodule detection and diagnosis capability compared with T2WI, and SPIO-enhanced imaging is a useful complement to the Gd-DTPA enhanced image for rat RN and HCC differentiated diagnosis.
     (2) Classifying rat and patient hepatic MR imaging using BP NN classifier based on texture feature and higher classification rate is obtained. Firstly, rat hepatic edge is automatically segmented by ISODATA algorithm, Otsu threshold algorithms and Iterative threshold algorithms on T1WI. The result shows that ISODATA algorithm is suitable for unknown category imaging automatical segmentation, and it is preparative for lesion region of interest (ROI) searching next step. Secondly, ROI of rat RN and HCC nodule SPIO-enhanced imaging and patient cirrhosis and normal liver parenchyma T1WI are cut manually and feature characteristics are extracted by Gray Level Coourrence Matrix (GLCM). Feature extraction program based on texture is chosen after comprehensive comparision. Six texture characteristic parameters including Angular Second Moment, Contrast, Correlation, Inverse Difference Moment, Entropy, and Variance are extracted according to the medical experience. For rat RN and HCC nodule SPIO-enhanced imaging, six texture characteristic parameters which are significantly different between two classes are chosen after statistical analysis. The result is same as clinical doctor's subjective visuality. Then a BP NN classifier is designed and classified.The independent test classification accuracy of the BP NN classifier is 91.67%. For patient cirrhosis and normal liver parenchyma T1WI, four texture characteristic parameters which are significantly different between two classes are chosen after statistical analysis. Then a BP NN classifier is designed and classified.The independent test classification accuracy of the BP NN classifier is 87.60%. It is concluded that the BP NN classifier based on texture feature is suitable for classification rat RN and HCC nodule SPIO-enhanced imaging and patient cirrhosis and normal liver parenchyma T1WI, but classification accuracy of the patient cirrhosis and normal liver parenchyma T1WI is low relatively.
     (3) For classification clinical patient cirrhosis T1WI, increasing the number of the texture feature parameters and optimizing the NN classifier by GA. Firstly, direction of GLCM is improved from one to four for extracting texture feature and 14 texture characteristic parameters of every direction are determined. Then feature is chosen by Box Plot. Among the 54 texture feature parameters,24 texture feature parameters which are preferably separability between cirrhosis and normal liver tissue are chosen. In the case of more input texture feature parameters, the BP NN classifier is optimized by GA and GA-BP algorithm is obtained. GA-BP algorithm is a mixed algorithm that the initial value of the BP NN classifier is optimized by GA. On ensuring the classification accuracy, stability and average convergence rate of GA-BP algorithm classifier is improved and the independent test classification accuracy of the GA-BP algorithm classifier is 95%.
引文
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