正常角膜和圆锥角膜的特征提取

Extracting features from normal corneas and keratoconus based on wavelet analysis

基于角膜测量仪器Corvis ST采集的图像视频,提出提取新特征参数以便准确区分正常角膜和圆锥角膜。首先对图像进行滤波、分割等预处理,检测角膜上下边界,并计算前角膜曲率值;用小波变换分析角膜曲率变化,获取与角膜运动趋势相关的特征,包括角膜运动的整体趋势和角膜振动的范数和标准差。然后,基于均方误差最小化法,提取特征参数,构建最优参数。最后,用支持向量机(SVM)对正常角膜和圆锥角膜进行分类。从频率的角度实施的实验显示角膜在基本运动趋势上存在着振动过程。此外,提出的参数优于形变幅度(DA)、峰值距离(PD)等传统参数,使准确度、灵敏度和特异性分别提高了10.2%,5.7%和6.9%。受试者工作特征曲线(ROC)下面积为0.948,接近于1。结果显示本文方法自动提取的特征参数可提高正常角膜和圆锥角膜区分的准确性,对临床诊断有辅助作用。

On the basis of video image captured by the cornea measuring instrument Corvis ST,this paper proposes an idea to improve the accuracy of distinguishing normal corneas from keratoconic corneas by extracting new feature parameters,Firstly,the original images were preprocessed by filtering and segmenting to detect the upper and lower boundaries of the cornea and calculate the curvature ofanterior cornea.Then,the change of corneal curvature was analyzed by wavelet transformation method to obtain features related to the trend of corneal movement,including the trend of the whole corneal motion as well the norm and the standard deviation of corneal vibration.Furthermore,the feature parameters were extracted in succession and the optimal parameter was obtained by the minimum mean square error algorithm.The Support Vector Machine（SVM）was finally applied to distinction of normal corneas from keratoconic corneas.The experiment results on the frequency indicate that there are corneal vibrations along with the basic movement process.Besides,the proposed parameters are better than traditional parameters such as Deformation Amplitude（DA）,Peak Distance（PD）at the highest concavity,which improves the accuracy,sensitivity and specificity by 10.2%,5.7% and6.9%,respectively.Moreover,the area under the receiver operating characteristic curve（ROC）is0.948,close to unity.The automatic extracted feature parameters in this paper are able to improve the accuracy of classification between normal and keratoconic corneas and contribute to the clinical diagnoses.