Cardiovascular disease accounts for around half the deaths in the Western world, with stroke being the third leading cause of death and disability worldwide 1. The predictive ability to identify which patients will have a stroke is poor. Current clinical practice for selecting which patients will have a carotid endarterectomy, to reduce the risk of stroke, relies on assessment of the degree of arterial lumen narrowing. Large European 2 and North American 3 clinical trials have demonstrated the benefit of carotid endarterectomy on groups of patients with severe stenosis. However the degree of stenosis is a poor predictor of individual stroke risk, and treatment of asymptomatic patients remains controversial 4. Recent studies on improved predictors of stroke risk have focused on the assessment of plaque morphology, composition, and the mechanical properties of the vessel wall and plaque deposit 56789101112.

Researchers have long hoped that the physical and mechanical properties of arteries may be of clinical value in the diagnosis and treatment of cardiovascular disease 1314. The carotid artery is a useful window for cardiovascular risk, with ultrasound able to provide unique information on morphological, hemodynamic and elastic properties 14. Numerous studies have shown that the elastic properties of arteries may be useful in the study of cardiovascular disease. The localized mechanical properties of carotid plaque and the vessel wall may be of particular value in identifying plaque vulnerable to rupture. The majority of clinical wall motion studies have derived elasticity parameters from measurements of the wall motion at a single location in vessels. Vessels have included the common carotid artery 1516, femoral artery 15, brachial artery 1718, cerebral arteries 19 and aorta 2021.

Various ultrasound techniques have been used to detect and track the vessel wall motion. Computational techniques have mainly been based on analysis of the B-mode greyscale images 182223, M-mode 16, analyses of the raw RF ultrasound data 24 and Doppler techniques 2025. Tissue Doppler Imaging (TDI) is a relatively new commercial technique, originally developed for imaging the myocardium, which has found increasing applications in echocardiography. TDI is basically a colour Doppler technique that has been optimised to provide images of tissue motion rather than blood flow, and is capable of high spatial and temporal resolution. The signal processing techniques employed to extract the tissue velocity information from the RF ultrasound data are typically based on time domain cross-correlation techniques 26 or autocorrelation techniques 27. Clinical applications of TDI of the myocardium, including theoretical, pathophysiological and methodological considerations are reviewed elsewhere 28. TDI has been applied to image arterial wall motion patterns of the common carotid artery (CCA) and to assess CCA stiffness by calculating distensibility as the change in normalised mean systolic distension relative to the change in blood pressure, and compliance as the change in cross-sectional area relative to pulse pressure 29.

Atherosclerotic plaque typically develops around the region of the internal and common carotid artery bifurcation. Knowledge of the differential wall motion displacements across the carotid bifurcation in normal and in diseased arteries may thus be useful in the study of atherosclerotic disease. An important clinical application is the identification of high-risk plaques that are vulnerable to rupture, leading to stroke. It is hoped the vulnerable plaque may be predicted by identifying pertinent features of the carotid plaque wall motion behaviour throughout the cardiac cycle. Initial clinical studies, using a TDI technique that was optimised to study carotid arterial wall motion (AWM) highlighted a number of case examples of carotid plaque disease suggesting the potential clinical use of AWM imaging 30.

The purpose of this study was to investigate the possible clinical value of these recent TDI techniques for AWM imaging in the vascular examination. In particular, this study assessed the dynamic wall motion deformation behavior of the carotid plaque and vessel wall. Arterial wall motion indices were quantified in a wide spectrum of normal and diseased carotid arteries to quantify baseline parameters, assess variability and help assess future clinical potential.

This is the first study to assess the clinical feasibility of TDI of carotid plaque wall motion using subjective and quantitative measures in a wide spectrum of normal and diseased arteries. Currently, clinically useful endpoints are based on morphological and hemodynamic properties of arteries. Extension of the ultrasound vascular examination to provide clinically useful vascular endpoints associated with the mechanical behaviour of carotid plaque may also be useful for characterising plaque.

Wall motion behaviour within a straight segment of the CCA typically displays synchronized, spatially homogeneous wall motion (Figure 1). This type of homogeneous wall motion is typically observed in normal straight segments of individual vessels and is in accord with previous studies on the normal CCA 31. An important potential application of measuring carotid plaque wall motion is to help identify the unstable plaque at high risk of rupture and consequent stroke. As plaque typically develops in the region of the carotid bifurcation, this study used a challenging imaging protocol that included the plaque (carotid bulb in normal arteries), the proximal segment of the ICA and the CCA. A variety of wall motion features were observed in a wide spectrum of normal and diseased arteries. The wall motion across the carotid bifurcation, even in vessels with no plaque may exhibit more complex behaviour than single vessels segments. Figure 2 shows an example of homogeneous wall motion across the bifurcation in a normal subject with no atherosclerotic disease. This is in contrast to more complex wall motion across the bifurcation (Figure 3) observed in a different subject, also with no evidence of atherosclerotic disease. This differential wall motion across the carotid bifurcation is not uncommon and represents a confounding factor in the use of our simple quantification measures for the differentiation of normal and diseased vessels. The variability may reflect true differences in wall motion between the different regions (ICA, carotid bulb and CCA). However, it is also likely, at least in part, to be caused by limitations and artefacts from various potential sources that we discus later.

The dynamic wall motion deformation behaviour appears to provide potentially useful information on the elastic nature of the vessels walls. A variety of common features were observed in patients with a wide range of atherosclerotic plaque disease. The clinical case studies illustrated in Figures 4, 5, 6 demonstrate a variety of pertinent qualitative and quantitative wall motion features related to the biophysics of arterial disease. These included examples of AWM constriction or stiffening associated with calcified plaque, constriction due to pressure drop through a severe stenosis (Venturi effect), high spatial gradient and intraplaque/vessel-interface inhomogeneities which were related to plaque type and severity. Pertinent features associated with clinical end points such as plaque development, plaque rupture and stroke are important and will require additional follow up studies.

It is also important to appreciate the potential problems and reliability of the technique. The clinical case examples in Figures 3, 7 and 8 showed examples of variable AWM behavior that did not appear to relate to plaque disease. These highlighted potential limitations of the technique and the use of simple quantification indices. Our simple AWM indices, which included the minimum and maximum wall dilations and spatial wall dilation gradients, showed a wide variation and generally had poor correlation with stenosis severity. The sources of error and variability may arise from various sources including the equipment, the operator, the patient and the limitations and design of our pilot study. A brief summary of some important considerations is given below.

Our pilot study demonstrates that the TDI Arterial Wall Motion imaging technique provides an impressive biomechanical window to atherosclerotic disease. It provides potentially useful information on the dynamic behavior of normal and diseased carotid arteries. Our initial clinical experience, using a challenging but realistic protocol for imaging carotid plaque wall motion, suggests the use of simple quantitative AWM measures may have limitations due to high variability. The technique must be robust and shown to be clinically useful if it is to play a major role in the vascular examination, where it is hoped that assessment of the mechanical properties of arteries, may help to provide a more rational basis for referring patients to carotid surgery.

None declared.

KVR conceived of the study, participated in its design and coordination, analyzed the data and drafted the manuscript.

TH, YS, MN and JW recruited patients, collected and reviewed the ultrasound data.

ARN recruited patients.

RBP and DHE participated in the study design and coordination.

All authors read, commented and approved the final manuscript.