At the 29th annual meeting of the Society of Interventional Radiology, held in Phoenix on March 25-30, 2004, Imaging of Carotid Disease was presented by Bruce Wasserman, MD, assistant professor, neuroradiology division, radiology department, Johns Hopkins School of Medicine, Baltimore. Wasserman reported that MRI holds great promise for imaging in carotid atherosclerosis. It is useful both for early lesion detection and for the evaluation of individual plaques in advanced disease. The mechanical and morphological characteristics of the plaque, along with the presence of active inflammation, are important predictors of the plaque’s likelihood of becoming dangerous. MRI can be helpful in this characterization, and may reach clinical use for this purpose in the near future.
DETECTION AND EVALUATION
Because early plaque forms eccentrically, Wasserman explained, it is important to image the vessel walls. As the interior of the artery narrows, blood velocity increases and vasodilation occurs. The lumen is normalized through vascular remodeling until plaque formation overwhelms this mechanism. Before this point has been reached, the lesion will be invisible angiographically, so vessel-wall imaging is needed to detect it.
Imaging the wall is also important at more advanced stages of disease, since even plaques that produce the same degree of lumen narrowing can behave differently. Stenosis of 80% might be created by each of two plaques, but one might be clinically silent and the other might cause frequent ischemic events. The degree of narrowing is less important than the structure and composition of the plaque.
Wasserman prefers to use the black-blood MRI technique to image plaque. The signal is suppressed in the lumen, highlighting plaque buildup in the adjacent vessel wall. A surface coil over the patient’s neck is used to permit high-resolution image acquisition.
In the carotid artery, steep gradients in blood velocity are created where flow divides. This creates shear stresses that cause longitudinal alignment of endothelial cells (which also develop tight intercellular junctions). These cells form a barrier to atherogenic particles, which are also likely to be removed by high-velocity blood flow. This pattern helps radiologists know where to look for plaque.
Early plaque formation is an inflammatory process involving increased endothelial permeability and macrophage recruitment. The same processes make it easier for gadolinium to enter the vessel wall, possibly permitting gadolinium enhancement of early plaque formation. Increased wall thickness and prolonged T2 relaxation might also be signs of vascular inflammation. Wasserman stated that MRI evidence of inflammation in aortic and carotid artery walls was correlated with the presence of C-reactive protein and other markers of inflammation.
The rupture of the fibrous cap separating the arterial lumen and the lipid core of the plaque is what begins a clinical event. The rupture exposes blood to high levels of tissue factor, a coagulant found in the lipid core at high concentrations. A clot forms and then embolizes in the carotid. The thicker the fibrous cap and the smaller the lipid core, the more stable the plaque. In addition, the cap’s composition, the presence of active inflammation, and the mechanical forces affecting the cap all play a part in its likelihood of rupture. Until a more refined model emerges, however, the large lipid core and thin fibrous cap remain the most useful risk indicators for MRI.
CLINICAL APPLICATION
Wasserman noted that research now in progress is likely to lead to clinical use of MRI for carotid atherosclerosis in coming years. In his research, he uses a single dose of gadolinium contrast to enhance the fibrous tissue and a 1.5-T magnet with a surface coil and a double inversion recovery fast-spin echo sequence for black-blood imaging. For 1.5-T imaging, Wasserman said that the limits of resolution had been reached at approximately 500 µm. In the carotid, however, a thin fibrous cap may be only 150 to 160 µm thick. The use of MRI to measure stresses on the plaque is now being validated, and markers other than gadolinium are under study.
MRI is now known to be useful not only for early plaque detection, but for the evaluation of advanced carotid-artery disease. Knowing where to look, using contrast enhancement, and understanding good flow-suppression technique are necessary. Clinical use is probable within the coming few years, with large multicenter studies now nearing completion. MRI can clarify the status of a single plaque, but a nuclear medicine study using a radioisotope can help identify the vulnerable patient, so the two techniques are likely to complement each other.
Kris Kyes is technical editor of Decisions in Axis Imaging News.
