The importance of plaque component classification and vessel wall quantification has been well established by several research groups (see Refs. [1-30]). Following are the two main reasons for this research:

1. Regression and progression of atherosclerosis: Direct plaque imaging is of potential use not only for diagnosis but also for monitoring response to treatment. Angiographic studies of progression and regression of atherosclerosis have been notoriusly poor in demonstrating changes in plaque burden, even when changes in clinical event rates have been markedly altered (see Brown et al. [6]). In a study of diet-/injury-induced atherosclerosis in rabbits, T2-weighted MRI identified regression of atherosclerosis 12-20 months after the withdrawal of the atherogenic diet (regression group). In contrast, lesion progression was documented in rabbits that were continued on the atherogenic diet (progression group). Helft et al. [25] showed that there was a significant reduction in the lipid

1 Biomedical Engineering Department, Case Western Reserve University, Cleveland OH, USA

2 Biomedical Engineering Department, Idaho State University, Pocatello, ID, USA

3 Department of Radiology, Case Western Reserve University, Cleveland OH, USA

Figure 9.1: Cross section of the artery showing lipids.

components of the plaque in the regression group and an increase in the progression group.

2. Importance of wall area: In a preliminary analysis, using PD-weighted and T2-weighted MRI, Corti et al. [27] illustrated a decrease in cross-sectional wall area in atherosclerotic segments of human aorta and carotid artery (by 8% and 15%, respectively) 12 months after the initiation of simvastatin. More importantly, there was no change in cross-sectional area of the arterial lumen. This emphasizes the importance of imaging the vessel wall directly and probably explains the limitations of coronary angiography in assessing response to treatment.

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