Identifying the vulnerable carotid plaque by means of dynamic ultrasound image analysis

Stroke is a global healthcare problem with very high rates of morbidity and mortality; therefore, early diagnosis and prevention are of paramount importance. Many strokes are caused by atherosclerotic plaques in the carotid arteries, and these are often assessed using ultrasound examinations that include the measurement of the degree of stenosis. However, despite the degree of stenosis being an important clinical marker of disease severity, there is an urgent need for additional parameters that can identify high-risk, vulnerable plaques, which may be more likely to cause stroke regardless of the degree of stenosis. This thesis describes the development of techniques for measuring plaque characteristics from ultrasound image sequences, testing the hypothesis that parameters obtained from these measurements can help identify vulnerable carotid plaques. Novel methods to track plaque boundaries in ultrasound image sequences were developed (Chapters 2 and 3). This allowed the dynamic assessment of plaque echogenicity (Chapter 3), a novel method of quantifying plaque surface irregularities (Chapter 4), and the investigation of arterial wall (Chapter 5) and plaque (Chapter 6) mechanics. In the penultimate chapter (Chapter 7), these parameters were integrated in the form of a carotid plaque risk index (CPRI) and its efficacy in predicting the presence of patient symptoms was assessed. The dynamic measures of plaque echogenicity and the novel plaque surface irregularity index correlated significantly with the presence of patient symptoms. The CPRI, which combines these parameters with the degree of stenosis, improved diagnostic accuracy compared to the degree of stenosis on its own, and led to a better separation of the symptomatic and asymptomatic patient groups. The methods for characterising plaque characteristics developed in this thesis could be valuable for identifying vulnerable carotid plaques. The risk index, if its efficacy is confirmed in subsequent clinical trials, may help reduce the incidence and burden of stroke.