Computational and Mathematical Methods in Medicine

Volume 2015 (2015), Article ID 419826, 8 pages

http://dx.doi.org/10.1155/2015/419826

## Identification of Mitral Annulus Hinge Point Based on Local Context Feature and Additive SVM Classifier

^{1}School of Computer and Communication Engineering, Changsha University of Science and Technology, Changsha 410114, China^{2}School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

Received 19 September 2014; Accepted 12 February 2015

Academic Editor: Yi Gao

Copyright © 2015 Jianming Zhang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The position of the hinge point of mitral annulus (MA) is important for segmentation, modeling and multimodalities registration of cardiac structures. The main difficulties in identifying the hinge point of MA are the inherent noisy, low resolution of echocardiography, and so on. This work aims to automatically detect the hinge point of MA by combining local context feature with additive support vector machines (SVM) classifier. The innovations are as follows: (1) designing a local context feature for MA in cardiac ultrasound image; (2) applying the additive kernel SVM classifier to identify the candidates of the hinge point of MA; (3) designing a weighted density field of candidates which represents the blocks of candidates; and (4) estimating an adaptive threshold on the weighted density field to get the position of the hinge point of MA and exclude the error from SVM classifier. The proposed algorithm is tested on echocardiographic four-chamber image sequence of 10 pediatric patients. Compared with the manual selected hinge points of MA which are selected by professional doctors, the mean error is in 0.96 ± 1.04 mm. Additive SVM classifier can fast and accurately identify the MA hinge point.

#### 1. Introduction

Congenital heart disease is one of the main reasons for death in children. The 3D shape and movement of the mitral apparatus are significant to analyze the function of left ventricular, diagnose mitral valve disease, and identify disorder of left ventricular [1–4]. There are some problems in surgical planning of mitral valve disease, including best surgical time and how to shape mitral valve. Therefore, analyzing the movement and shape of mitral apparatus with advanced computer technology and imaging technology has important clinical value and social value.

Precise positioning the hinge point of mitral value is helpful for modeling, motion tracking, and multimodalities registration of cardiac images. The vague and incomplete ventricular structure in ultrasound images due to the heavy noise, low resolution, and limited imaging range in real time echocardiography causes great difficulties to identify the mitral value manually and automatically. Nevo et al. [5] introduces an automated tracking algorithm using multidimensional dynamic programming which tracks the hinge point of mitral value leaflet in 2D echocardiographic images. Takemoto et al. [6] introduces an automated mitral annular tracking method. It adopts a partial shape constraint contour model to track and fit the ambiguous ventricular boundary and recognizes hinge point of mitral value using pattern matching algorithm. Veronesi et al. [7] tracks the mitral annular in 4D echocardiographic images combining optical flow method with block matching method. Due to the cumulative errors, the results require manual correction. Schneider et al. [8] use constrained optical flow combined with graph cut [9] and a valve state predictor to segment mitral annulus from four-dimensional ultrasound images.

This paper introduces a hinge point of mitral annulus identification algorithm based on additive SVM classifier [10, 11]. The optimized additive SVM classifier which can more quickly and efficiently classify test sample gets the same classification accuracy as classic SVM classifier. It is difficult to design a feature for echocardiographic image because of the heavy noise and low resolution. The typical global feature such as local binary pattern (LBP) [12, 13] and histogram cannot specify the hinge point in the whole cardiac structure. Spatial relationships of atrium and ventricle are fixed in the echocardiography. Therefore, in this work, a local context feature is obtained for subsequent classification of MA hinge point candidates for intracardiac structures in echocardiography. Ideally, every pixel in the neighborhood can be put into context. However, this would generate a large feature space. This paper designs a local context feature which sparely samples the gray value of the context locations on eight directions in 45-degree intervals. Reasonable results will be achieved after applying the additive SVM classifier on this local context feature. The rest of this paper is organized as follows. Section 2 introduces additive SVM classifier. In Section 3, we describe the local context feature. Section 4 gives an improved method. In Section 5, we present the flow of classification. Section 6 shows some experimental results that demonstrate the effectiveness of this algorithm. Finally, we conclude in Section 7.

#### 2. Additive SVM Classifier

SVM and boosted decision tree [14, 15] are the two main methods in target detection and multitargets recognition. Classifiers based on boosted decision trees have faster classification speed, but they are significantly slower to train. Furthermore, the complexity of training grows exponentially with the number of classes. The linear SVM is efficient during training on a given feature space. It can be used in real-time applications for low memory requirements and fast classification speed. Although the kernel trick is introduced to handle nonlinear problems in SVM, its complexity is much higher than linear SVM.

The linear SVM is more efficient, but many nonlinear kernels can get better results in pattern classification tasks due to the nonlinear distribution of features. Some popular nonlinear kernels which are based on histograms of low-level features like color and texture of the image use a kernel derived from histogram intersection or chi-squared distance to train a SVM classifier. To evaluate the classification function, the test histogram is compared with every support vector histogram. Maji et al. [10, 11] present a method which can efficiently compute the classification function based on histograms. This optimized method improves additive kernel SVMs significantly and can be used in any additive kernels.

##### 2.1. Histogram Intersection Kernel SVMs

Given training set , class label ; vector . To find the hyper plane to separate the sample set in linear problem, the minimization function can be written aswhereand is the slack variable, is used to maximize the distance between support vectors and hyper plane, is used to minimize the error rate, is the weight between maximized distance and slack constraints, is the normal vector to the hyperplane, and determines the offset of the hyperplane from the origin along the normal vector . The kernel is the inner product in high dimension. The maximized duality function iswhere is Lagrange multiplier, , . Decision function is , whereAnd is the support vector. Therefore, classifying a test feature requires times calculation of kernel function and stores support vectors. Assuming that the complexity of decision function is , the complexity of classification of one test feature is . As to linear kernel function , its decision function is where . So the complexity of linear SVM is .

With similarity of feature such as boundary, color can be represented as histogram which regularly uses histogram intersection as its evaluation of similarity. The histogram intersection kernel iswhere is histogram feature. The decision function isThe complexity of (6) is still . The key property of intersection kernels is that the order of summing can be exchanged. So, (6) can be converted to the following:Function can be represented as the sum of 1D functions , where

So as to make the complexity of to , let denote the sorted in increasing order; the corresponding and are and . If , then . Otherwise, let be the largest integer satisfying . The function is transformed towhere , . Functions and which only depend on the support vectors and can be computed after SVM model is trained. Binary search is adopted to get and reduces the complexity.

#### 3. Local Context Feature

The general image detection operator such as the Sobel operator and the Laplace operator cannot be applied to ultrasound images due to the heavy noise and blurred boundary. Context is the relationship with the neighbors which can be represented as a certain range of neighbors of a pixel in image processing. This paper introduces a local context feature which sparsely sample [16] the neighbor pixels of a pixel. Sampling sparsely can decrease the dimensions and shorten the training time. Because of the fixed spatial distribution of cardiac tissues in ultrasound images, the local context feature can quantify the position relationship between cardiac tissues.

As Figure 1 shows, red points represent the sampling points. This feature sampling at eight directions and the intervals between sample points largen as the distance to the center point lengthens. The points nearer to the center point contain more information about the center point, so most feature samples points are in near range. Taking the size of cardiac ultrasound image into account, the max sampling distance sets as 29 and the sampling position at each direction is . Because of the heavy noise in echocardiography, the gray value of sampling point will make training and classification features inaccurate, which will reduce the recognition rate. In order to eliminate the error caused by the noise, we apply mean filtering to the sampling points when sampling. Because the larger average template cannot make the sampling gray value accurately reflect the information of sampling points, we set the average of 3 3 neighborhoods as the value of each sampling point. So one pixel can get a 65-dimension feature and the local feature is extracted fast.