Cath Lab Digest

Acute coronary syndromes (ACS) and ischemic strokes develop suddenly and often unpredictably in patients with vascular disease. In the majority of patients, these clinical scenarios result from either rupture of a thin-cap fibroatheroma or superficial erosion of an atheroma.¹ An intraluminal thrombus then forms on the damaged lesion, possibly embolizing and resulting in decreased blood flow within the artery leading to ischemia and clinical instability. In the coronary circulation, the result of thrombus and embolus formation is myocardial ischemia, infarction, or death, while in the carotid or aortic circulation, an ischemic stroke is the consequence. In the peripheral vasculature, distal ischemia may result. Such rupture-prone lesions, mostly non flow-limiting plaques, have been observed in coronary arteries, the aortic arch, carotid artery bifurcation, proximal segment of the renal, superior mesenteric, and celiac arteries, the aortic bifurcation, and iliac and femoral arteries.²

At the current time, there is no available diagnostic approach to predict the presence of unstable atherosclerotic lesions. Angiography, although excellent for demonstrating the presence of flow-limiting lesions or moderate atherosclerosis, is limited, as it demonstrates only the arterial lumen and not the arterial wall. Also, early atherosclerotic lesions or lesions with expansive remodeling cannot be visualized by angiography. These lesions are more likely to be vulnerable, as several studies have shown that the majority of myocardial infarctions (MI) are caused not by the most angiographic stenotic lesion, but lesions intermediate in severity (e.g., 50–70% in diameter reduction).

Intravascular ultrasonography is the standard in determining the presence and extent of atherosclerotic disease, but has generally proven inadequate in determining lesion composition and predicting future clinical developments. As a result, new diagnostic approaches have been developed to locate such potentially vulnerable plaques prior to plaque instability and a subsequent clinical event. These technologies, which can identify the underlying pathophysiologic substrate, may be more helpful in correlating plaque biology with future clinical developments. Intravascular magnetic resonance imaging (IVMRI) is one such device.

Attraction of MRI for Evaluation of Plaque Composition
Magnetic resonance imaging (MRI) interrogates the differential biophysical and biochemical response of protons following application of a transient electromagnetic radiofrequency (RF) pulse in the setting of a strong static magnetic field. MRI is a powerful tool to determine the presence of lipids within the arterial wall or, when used in combination with local delivery of contrast agents, to determine the presence of specific cell types associated with plaque instability, such as thrombi,^3,4 activated macrophages,⁵ or tissue factor.⁶ Noninvasive, high-resolution, multicontrast MRI has been used to assess atherosclerotic lesion composition, as well as determine fibrous cap thickness, necrotic core size and fissures within the fibrous cap of atherosclerotic lesions^7–10 and has been used clinically to identify the presence of ruptured fibrous caps that are associated with transient ischemic attacks or stroke.¹¹

However, noninvasive MRI has not been shown to effectively and reproducibly evaluate coronary artery lesion composition. The small volume of the typical coronary plaque and the tortuous and irregular course of the vessels make imaging of coronary arteries difficult. In order to prevent motion artifacts, MRI requires both respiratory and cardiac gating. Furthermore, as the distance increases between a MRI coil and the interrogated structure (in this case an artery), there is a corresponding decrease in the signal to noise ratio (SNR), and a consequent reduction in the image resolution. The deeper location of the coronary arteries compared to the surface of the chest (4–10 cm), and the difficulty of optimal receiver coil placement makes attaining a sufficient SNR a major challenge for coronary MRI. To provide a possible solution, a novel intravascular MRI catheter has been developed within which the magnets, RF transmitters, and receivers are miniaturized. The IVMRI holds promise in the in vivo evaluation of lipid-rich, potentially unstable vascular lesions.

Technical Properties of the Intravascular MRI Catheter
The intravascular MRI system is an integrated, self-contained MRI probe positioned on the tip of a vascular catheter attached to a portable control unit. There are no external magnets or coils. Hence, the catheter can be used in the catheterization suite rather than within a MRI magnet. Local static magnetic field gradients are generated at the site of measurement, which is responsive to the diffusion properties of the analyzed vascular tissue.

IVMRI is currently designed to determine the presence of lipids with the arterial wall, since MRI is particularly adept at differentiating between fibrous and lipid-laden tissue. Insofar that the fibrous cap and the normal medial layer possess similar biophysical properties, IVMRI cannot differentiate the two. However, lipid-laden tissue can easily be differentiated from normal, fibrous, and calcific tissue.