Atherosclerosis imaging using MR imaging: current and emerging applications

Strategies to reverse atherosclerosis: an imaging perspective

Several treatment strategies, including lowering low-density lipoprotein cholesterol with intensive statin therapy, reducing triglycerides with fibrates, and raising high-density lipoprotein cholesterol with nicotinic acid, have the potential to induce atherosclerosis regression. Atherosclerosis imaging techniques including intravascular ultrasound, carotid ultrasound to measure carotid intima-media thickness, and cardiovascular MRI are established modalities for describing longitudinal changes in the quantity and quality of atherosclerotic plaque. An increasing number of clinical trials are using radiologic measures of subclinical atherosclerosis as surrogate end points in lieu of the traditional "hard" end points of myocardial infarction and death. This approach has great appeal, as improvements in atherosclerosis imaging now enable the characterization of early atheromas and positive remodeling within the vessel wall before the plaque becomes obstructive. Additional prospective data correlating these surrogate end points with hard outcomes are needed to determine whether atherosclerosis regression will be the major determinant of future treatment strategies.

Clinical and research applications of carotid intima-media thickness

Cardiovascular imaging modalities such as coronary computed tomography, carotid ultrasonography, and cardiovascular magnetic resonance imaging are increasingly being used to measure cardiovascular disease progression. Imaging measures, most notably carotid intima-media thickness (CIMT), are being applied as surrogate markers for clinical end points such as myocardial infarction and death in clinical trials. Clinicians and their patients are faced with the challenge of evaluating these imaging measures for their efficacy and practicality in clinical practice, as well as in clinical trials. The investigators determined from a review of clinical trials and guidelines that CIMT measurement may be useful in evaluating cardiovascular disease risk in select patient populations but may not always be an appropriate surrogate for clinical end points. In conclusion, although CIMT has clear advantages over alternative cardiovascular imaging modalities, ultimately, prospective trials comparing the effectiveness of CIMT as a predictive tool of cardiovascular risk with that of other novel markers would best direct clinical recommendations for this imaging measure.

Imaging of atherosclerosis using magnetic resonance: state of the art and future directions

Atherosclerosis is the leading cause of morbidity and mortality in industrialized societies, and its incidence is projected to increase in the future. Because the atherosclerotic process begins in the vessel wall, the focus of cardiovascular imaging is shifting from the arterial lumen to imaging of the vessel wall, with the goal of detecting preclinical atherosclerosis. MRI, because of its high resolution, three-dimensional capabilities, noninvasive nature, and capacity for soft tissue characterization, is emerging as an important modality to assess the atherosclerotic plaque burden in the arterial wall and can monitor atherosclerosis in different arterial beds, including the carotid arteries, aorta, and more recently, the coronary arteries. Furthermore, it has also been successfully utilized to monitor plaque regression following therapeutic interventions. Finally, the emergence of high-resolution MRI and development of sophisticated contrast agents offers tremendous promise for in vivo molecular imaging of the atherosclerotic plaque.

Development of contrast agents targeted to macrophage scavenger receptors for MRI of vascular inflammation

Atherosclerosis is a leading cause of death in the U.S. Because there is a potential to prevent coronary and arterial disease through early diagnosis, there is a need for methods to image arteries in the subclinical stage as well as clinical stage using various noninvasive techniques, including magnetic resonance imaging (MRI). We describe a development of a novel MRI contrast agent targeted to plaques that will allow imaging of lesion formation. The contrast agent is directed to macrophages, one of the earliest components of developing plaques. Macrophages are labeled through the macrophage scavenger receptor A, a macrophage specific cell surface protein, using an MRI contrast agent derived from scavenger receptor ligands. We have synthesized and characterized these contrast agents with a range of relaxivities. In vitro studies show that the targeted contrast agent accumulates in macrophages, and solution studies indicate that micromolar concentrations are sufficient to produce contrast in an MR image. Cell toxicity and initial biodistribution studies indicate low toxicity, no detectable retention in normal blood vessels, and rapid clearance from blood. The promising performance of this contrast agent targeted toward vascular inflammation opens doors to tracking of other inflammatory diseases such as tumor immunotherapy and transplant acceptance using MRI.