MR imaging of the vessel wall

Since the risks for thrombosis are more dependent on plaque composition than on the degree of luminal narrowing, the radiological assessment of atherosclerosis should extend beyond mere depiction of the arterial lumen. High-resolution MRI of the vessel wall can provide important information about the individual makeup of atherosclerotic plaques, potentially enabling early detection and characterization of lesions before narrowing of the vessel lumen occurs. The MR-based assessment of the vessel wall with sufficient spatial resolution and image contrast, however, remains challenging. Requirements include high signal-to-noise ratio, high contrast-to-noise ratio, good signal penetration depth and homogeneous signal throughout the vessel under investigation, as well as imaging protocols encompassing various contrast weightings. Numerous dedicated radiofrequency (RF) coils have been developed to achieve these goals employing either external surface phased-array coils, or alternatively, utilizing intravascular coils to image the vessel wall from inside the vessel and thus being invasive. For the non-invasive approach of imaging with surface coils, the carotid and the right coronary arteries have been favoured since they are of critical importance and since they are relatively superficial structures, accessible from the outside. To detect the early development of plaque and visualize it globally rather than locally, intravascular contrast agents on the basis of ultrasmall particles of iron oxide can be used as a marker of macrophage activity within the plaque. In the long run, it appears likely that the combination of luminal MR angiography with the administration of susceptibility-based contrast agents and subsequent high-resolution MR of detected atherosclerotic lesions with dedicated RF coils could evolve as the diagnostic concept of choice for the assessment of atherosclerotic disease.