Intravascular MRI: A Novel Tool in the Assessment of High-Risk Plaques

Atherothrombosis is a systemic disease of large- and medium-sized arteries, including the coronary, aorta, and peripheral arteries. The clinical manifestations depend on the size of the vessel and the regional circulation involved and include coronary artery disease, stroke, and peripheral vascular disease. A paradigm shift is occurring, with a change in focus from the assessment and treatment of luminal narrowing towards greater understanding of the vascular biology in the arterial wall that leads to plaque vulnerability. High-risk or vulnerable plaques are characterized by the presence of a thin, fibrous cap (typically < 65 µm), lipid-rich necrotic core, inflammatory cell infiltrates, and paucity of smooth muscle cells at the cap shoulders, with evidence of neovascularization, calcification, and intraplaque hemorrhage in advanced plaques.¹ In this issue of Vascular Disease Management, Wilensky et al discuss the role of magnetic resonance imaging (MRI) with emphasis on a novel approach to high-risk plaque evaluation with intravascular MRI (IVMRI).

MRI perhaps provides the most promise in noninvasive, high-risk plaque imaging. The high spatial resolution achieved, ability to characterize plaque composition, and use of contrast to identify inflammatory, neovascular, and thrombotic components of plaque are the strengths of the technique. However, noninvasive MRI visualization of coronary arteries is limited currently by the small size of the coronary arteries, arterial motion, and deep location. IVMRI using intravascular coil can potentially provide superior resolution to standard MRI, and ex vivo aortic plaque characterization demonstrated improved vessel wall characterization, with sufficient resolution to discern the intima, media, and adventitia, including the fibrous cap and lipid core.^2,3 However, in vivo application is hindered by the significant fall-off in signal to noise as the distance between the coil and the artery increases, necessitating close apposition between the two, potential effects of flowing blood to plaque characterization, image quality degradation when the coil moves off from the external magnetic field,² and the potential for local vessel wall heating with <i>in</i> vivo coils, particularly at high field strengths.⁴

Wilensky et al describe a novel IVMRI system that consists of a catheter containing an integrated MRI probe attached to a portable unit. Since it has no external coils or magnets, the system is portable, making it feasible for use in the catheterization laboratory. IVMRI makes use of the differential water diffusion coefficients within the atherosclerotic plaque lipid core compared to the fibrous cap and the medial smooth-muscle layer and, as such, can determine the location and extent of lipid infiltration in the vessel segment in question. Phase I and II human clinical trials have shown the safety and feasibility of this technique in the evaluation of high-risk plaques in the coronary system. It can also be used to effectively characterize lipid-rich, high-risk plaques in the peripheral circulation. It can potentially be applied as a tool in stent selection, optimal stent apposition and expansion, and prevention of distal embolization.

IVMRI using a probe provides a simplified spatial representation of its lipid-rich component and does not reflect the actual plaque morphology. The probe cannot differentiate between the fibrous cap and the normal medial layer, due to similar water diffusion coefficients. Other limitations include the invasive nature of the procedure, the size of the catheter, the need to transiently compromise blood flow with balloon inflation during catheter stabilization, and the need to mechanically rotate the catheter for circumferential sector assessment.⁴

In summary, IVMRI, using an integrated, self-containing MRI probe positioned at the tip of a catheter provides a novel tool in the assessment of high-risk plaques. Despite its current limitations, it can be used as an effective tool to assess lipid content of plaque in multiple vascular territories. How it is going to perform in this era of invasive imaging, being done mostly with intravascular ultrasound (IVUS), remains to be seen. Wilensky et al should be congratulated for their role in the development of this emerging technology.