Atherosclerosis imaging: intravascular ultrasound

Most acute coronary syndromes result from the rupture or erosion of high-risk plaques. Clinical imaging studies have shown that atherosclerotic plaque formation and rupture are widespread processes that are often asymptomatic. The rationale for atherosclerosis imaging is the in-vivo identification of high-risk lesions, which may subsequently lead to prevention of future cardiovascular events. Although intravascular ultrasound (IVUS) imaging studies demonstrated that echolucent appearance of the plaque and expansive (positive) remodelling are associated with unstable clinical presentation, these characteristics were not adequate for accurate plaque characterisation. Recent technical developments in ultrasound equipment and analytical methods, utilising several characteristics of the digitised ultrasound signal with radiofrequency analysis and elastography, promise accurate tissue characterisation. Other imaging modalities, including optical coherence tomography, also contribute to a more precise characterisation of the composition of atherosclerotic plaques. A non-imaging approach is the focal assessment of temperature differences using sensitive intravascular thermography catheters, presumably reflecting focal inflammatory changes of vulnerable lesions. Although the histological characteristics of the atheroma are critically important in the sequence of events leading to acute coronary syndromes, the clinical relevance of identifying these characteristics is not yet clear. There is increasing evidence that identifying and treating individual culprit lesions may not be enough to prevent the ischaemic cardiac events in most patients, because the acute coronary syndrome is not a disease of a single site or a few discrete segments, but rather a systemic disease that involves the entire coronary tree. In addition to detection and quantitation of early coronary atherosclerosis and disease activity, accurate and reproducible methods could help to identify high-risk patients and allow serial monitoring during various therapeutic interventions. Serial IVUS imaging makes it possible to visualise the vessel wall that harbours the atheroma at different time points. Typically, serial IVUS allows the assessment of the percentage change in atheroma volume, with considerable statistical power to detect small changes. Using this methodology, aggressive lipid lowering by a high-dose statin agent has been shown to stop the progression of atherosclerosis, and a new mutant high-density lipoprotein complex was found to be effective in regressing atheroma burden. Although intravascular ultrasound is very accurate for quantification of atheroma burden, widespread application and accurate and reproducible non-invasive imaging modalities are needed for large-scale risk assessment algorithms. Cardiovascular computed tomography is at the forefront of the non-invasive imaging modalities. Future prospective imaging studies will be necessary to identify focal or systemic characteristics of high-risk lesions and to demonstrate the relationship between plaque burden, biochemical markers and clinical events.