Fully automated segmentation and tracking of the intima media thickness in ultrasound video sequences of the common carotid artery

The robust identification and measurement of the intima media thickness (IMT) has a high clinical relevance because it represents one of the most precise predictors used in the assessment of potential future cardiovascular events. To facilitate the analysis of arterial wall thickening in serial clinical investigations, in this paper we have developed a novel fully automatic algorithm for the segmentation, measurement, and tracking of the intima media complex (IMC) in B-mode ultrasound video sequences. The proposed algorithm entails a two-stage image analysis process that initially addresses the segmentation of the IMC in the first frame of the ultrasound video sequence using a model-based approach; in the second step, a novel customized tracking procedure is applied to robustly detect the IMC in the subsequent frames. For the video tracking procedure, we introduce a spatially coherent algorithm called adaptive normalized correlation that prevents the tracking process from converging to wrong arterial interfaces. This represents the main contribution of this paper and was developed to deal with inconsistencies in the appearance of the IMC over the cardiac cycle. The quantitative evaluation has been carried out on 40 ultrasound video sequences of the common carotid artery (CCA) by comparing the results returned by the developed algorithm with respect to ground truth data that has been manually annotated by clinical experts. The measured IMT_mean ± standard deviation recorded by the proposed algorithm is 0.60 mm ± 0.10, with a mean coefficient of variation (CV) of 2.05%, whereas the corresponding result obtained for the manually annotated ground truth data is 0.60 mm ± 0.11 with a mean CV equal to 5.60%. The numerical results reported in this paper indicate that the proposed algorithm is able to correctly segment and track the IMC in ultrasound CCA video sequences, and we were encouraged by the stability of our technique when applied to data captured under different imaging conditions. Future clinical studies will focus on the evaluation of patients that are affected by advanced cardiovascular conditions such as focal thickening and arterial plaques.

I. Introduction

ATHEROTHROMBOTIC events, myocardial infarction, and stroke are responsible for approximately 35% of the total mortality in the western world and are the leading causes of morbidity burden world-wide. The first indication of cardiovascular disease is a thickening of the intimal and medial layers of the arterial wall [1], commonly known as intima media thickness (IMT). The intima media complex (IMC) is best visualized in longitudinal sections of the common carotid artery and, as shown in Fig. 1, it is composed of two quasi-parallel lines that represent the lumen intima (LI) and the media adventitia (MA) interfaces. The thickening of the intimal and medial layers is caused by inflammatory-fibroproliferative responses to various forms of insult. It involves lipid accumulation and the migration and proliferation of many cells in the subintimal and medial layers, which result in the formation of plaques. It is the rupture of such plaques that causes myocardial infarcts (heart attacks), cerebrovascular events (strokes), peripheral vascular disease (gangrene), and kidney infarcts [2]. To emphasize the impact of the IMC thickening process on the incidence of cardiovascular events, the Rotterdam study indicates that the risk of myocardial infarction increases 43% per standard deviation increase (0.163 mm) in common carotid IMT [3]. The main conclusions resulting from this study were supported by other independent investigations, which reveal that an IMT higher than 0.9 to 1.0 mm indicates a potential atherosclerotic disease [1], [3], [4]. Hence, the robust segmentation and measurement of the IMT has a considerable impact in the early diagnosis of atherosclerosis, prognosis prediction, and in the monitoring of responses to lifestyle and prescribed pharmacological treatments. In addition to the IMT estimation, changes in the mechanical properties of the arterial wall are of interest because they also have the potential to indicate the existence of early cardiovascular diseases. These changes can be detected by analyzing the arterial wall stiffness (or elasticity) using techniques such as diameter change estimation, artery distensibility, or strain imaging [5]–[8]. Fig. 1. (a) First frame of a cardiac cycle displaying a longitudinal section of the carotid artery. (b) Detail of the far wall that shows the two anatomical boundaries that compose the intima media complex (IMC), as marked by a medical expert. The intima media thickness (IMT) value is given by the distance between the two interfaces: lumen-intima (LI) and media-adventitia (MA). For this image, we have recorded the following values: mm, mm, mm, and mm. <inline-graphic xlink:href="6396496-graphic-1-source.tif" xmlns:xlink="http://www.w3.org/1999/xlink"/>