Abstract-In this paper, we investigate the possibility of using the fractal dimension to characterise carotid atheromatous plaques from B-mode ultrasound images. The images were obtained from ten symptomatic and nine asymptomatic subjects. Symptomatic subjects included patients with previous history of cerebral events, whereas asymptomatic ones had no evidence of any cerebral symptoms prior to the time of the investigation. For each subject, a sequence of images was collected corresponding to 2-3 cardiac cycles. The boundary of the atheromatous plaque was defined by an expert in three different images of each sequence, corresponding to systole, diastole and a random phase of the cardiac cycle. The fractal dimension of each plaque was estimated using a new method, namely the k-th nearest neighbour method. The results showed that the values of the fractal dimension in the symptomatic group were significantly higher than those in the asymptomatic group. These results suggested that the fractal dimension, estimated from B-mode ultrasound images, could be used to discriminate between symptomatic and asymptomatic carotid atheromatous plaques. Keywords -B-mode ultrasound, carotid, atherosclerotic plaque, fractal dimension, kth nearest neighbour method
I. INTRODUCTIONUltrasound imaging of the carotid artery is widely used in the diagnosis of carotid atherosclerosis as it allows noninvasive assessment of the degree of stenosis as well as of plaque morphology, including plaque homogeneity/ heterogeneity, plaque echogenicity/echolucency, plaque texture and plaque surface characteristics. It has been demonstrated that the possibility that a carotid plaque produces a cerebral event (symptom), i.e. its instability, may be determined by the degree of stenosis [1], with plaques causing high degrees of stenosis more likely to produce symptoms.However, carotid plaques with relatively small degrees of stenosis have been reported to cause symptoms. Furthermore, due to the fact that the majority of symptoms occur in previously asymptomatic subjects, it is important to study additional parameters that may contribute to recognise and treat subjects (especially asymptomatic ones) at high risk of cerebral events.Plaque morphology, determined from ultrasound images of the carotid artery, may be related to the risk of clinical events. Homogeneous plaques are characterised by uniformly high-or medium-level echoes and are associated with stable plaques; heterogeneous plaques are associated with advanced stages of carotid plaque lesion [2]. Echogenic plaques reflect strongly the ultrasound signal, whereas echolucent ones have less reflecting ability. It has been shown that echolucent plaques, as evaluated by B-mode ultrasound, are more likely to lead to the development of neurological events than echogenic ones [3]. Analysis of digital images of carotid plaques allows assessment of plaque texture. First and second order statistical features extracted from carotid plaques may provide useful information about the correlation between texture an...