Automatic measurement of atherosclerotic plaque burden in ultrasound images of the carotid artery

The aim of the present work is the development of a computer-assisted diagnosis system for the automatic measurement of plaque burden in ultrasound images of the carotid artery. Atherosclerosis is a vascular disease that constitutes one of the main causes of death and permanent disability in developed countries. In most cases, it is asymptomatic and can only be diagnosed when it leads to a cardiovascular or cerebrovascular event, such as heart attack or stroke, respectively. It is of great interest to find markers with high predictive value that allow an early detection and diagnosis of atherosclerosis. One of these markers is plaque burden, which measures the amount of plaque present in diseased arteries, and therefore is a useful marker for the diagnosis of atherosclerosis in its later stages, allowing the monitoring of the progression of the disease and its response to different therapies and treatments. Plaque burden is usually measured in ultrasound images of the carotid artery. For this, a prior segmentation of the atherosclerotic plaques is required. In the present work, plaque burden is expressed as the sum of the areas of the atherosclerotic plaques segmented in a two-dimensional B-mode ultrasound image of the common carotid artery. For this segmentation, a novel method is proposed which is divided in two main steps: 1) segmentation of the intima-media layer of the carotid wall and 2) plaque delimitation using the segmented contours of this region. For the segmentation of the carotid wall a previously developed algorithm is used as reference and modified to improve the segmentation of the intima-media contours in images affected by speckle noise. The following optimizations are introduced: use of an alternative method for lumen detection based on local phase symmetry patterns at appropriate scale of analysis; relaxation of the constraints imposed to the contour; use of a new membership function for the fuzzy classification of edges that uses more image information, such as the edge strength; local estimation of the strength of false edges caused by noise in the lumen, calculated through robust statistics, to help defining the penalty and gain terms of this function. x For plaque segmentation, the proposed method uses objective criteria to define atherosclerotic plaque proposed in a medical consensus. The segmented intima-media contours are used to calculate measures of the local thickness of the arterial wall. These measures are compared with reference thresholds specific of each criteria to evaluate the presence of atherosclerotic plaques along the carotid wall. The proposed method for plaque segmentation was tested using a dataset of 2D Bmode ultrasound images of the common carotid artery, in comparison with manual segmentations of atherosclerotic plaques and the contours of the intima-media region. The method demonstrates a good performance when the intimamedia layer is correctly segmented, showing plaque segmentations comparable to those obtained manually. A graphical user interface was developed for the calculation of the image plaque burden. It allows to select the image, automatically segment the atherosclerotic plaques and quantitatively characterize the length and area of each plaque. Plaque burden is expressed as the sum of the areas of all the plaques in the image. The average and maximum value of the local intima-media thickness are also presented.