Atherosclerosis is the basis of the majority of carotid artery disease which, via occlusive/stenotic disease and subsequent thromboembolic events, results in end organ (brain) damage. An element of the atheromatous plaque now becoming appreciated as potentially important in progression and rupture is intraplaque hemorrhage (IPH). Applying T1-weighted magnetic resonance imaging (MRI) to patients suspected of suffering deep vein thrombosis or pulmonary embolism, the identification of high signal within the deep veins of the legs or the arteries of the lungs was shown to accurately identify intravascular thrombus. At the same time as intravascular methemoglobin was under investigation there was increasing interest in using MRI to directly image atherosclerotic disease within the vessel wall. Exploiting the already proven ability of MRI to generate high signal from methemoglobin, the possibility exists to specifically target hemorrhage within plaque using this technique.

This chapter introduces the state-of-the-art noninvasive magnetic resonance imaging (MRI) techniques that are used to monitor atherosclerosis of the carotid artery. High-resolution MRI is an ideal plaque imaging technique because it is noninvasive and able to create excellent soft tissue contrast and distinguish flowing blood from surrounding stationary tissues. Multicontrast weighted imaging protocol provides an oblique view of the carotid artery to better visualize the location of the carotid bifurcation and to demonstrate plaque distribution. The objective of the American Heart Association (AHA) histological classification of atherosclerosis, first published in 1995, was to provide a clinically relevant categorization of human atherosclerotic lesions based on their histological composition and structure. Major plaque components include fibrous connective tissue, the lipid-rich necrotic core, intraplaque hemorrhage, and calcification. MRI is capable of identifying many of the key vulnerable plaque features defined by the expert panel with a high level of accuracy and reproducibility.