High-resolution magnetic resonance imaging: an emerging tool for evaluating intracranial arterial disease.

A therosclerosis occurs in diverse vascular beds and may result in tissue ischemia. Current understanding of ath-erosclerotic disease has been advanced by imaging techniques , such as high-resolution magnetic resonance imaging (MRI; HRMRI)studies of the coronary and carotid arteries. In these vessels, atherosclerotic plaque components can be visualized to risk-stratify patients, select treatments, and advance our understanding of atherosclerosis pathophysiology in vivo. These imaging techniques are now being applied to evaluate intracranial arterial disease, both atherosclerotic and non-atherosclerotic. This review highlights the mechanisms by which intracranial atherosclerotic disease causes ischemia, the potential of HRMRI for identifying intracranial arterial pathology, the limitations of HRMRI in the intracranial circulation , and future applications of HRMRI for intracranial atherosclerotic disease. Image quality in MRI depends on several factors (eg, slice thickness, field of view, signal-to-noise ratio, matrix size, and magnetic field strength), but the term HRMRI is not well defined. In this review, the operational definition of HRMRI is limited to magnetic resonance acquisitions using clinically available 1.5 to 3.0 T magnetic field strengths targeted to intracranial arterial pathology that are of sufficient quality to visualize the arterial wall, separate from the lumen, of the proximal circle of Willis vessels. HRMRI can be accomplished at 1.5 T by limiting the field of view to focus on a single vessel or point of interest, but higher field strength at 3.0 T has many advantages over conventional (1.5 T) MRI. Image acquisition is faster 1 and there are increased signal-to-noise 2 and contrast-to-noise ratios, with better image quality 3 for black-blood imaging. The increased signal and contrast that 3 T provides improves the detection of complex atherosclerotic plaque 4 and can identify plaque components in larger arteries. 5 Two-dimensionally acquired HRMRI is time consuming and must be monitored by a neuroradiologist to ensure adequate sampling of the lesions of interest. HRMRI using 3-dimensional (isotropic) acquisitions permits imaging of intracranial atherosclerotic disease (ICAD) with shorter scan times and generates better-quality images. The primary mechanisms by which ICAD results in ischemic stroke are plaque rupture, occlusion of small penetrating arteries, and hypoperfusion. Plaque rupture exposes the thrombogenic core to clotting factors, and the resulting thrombus either occludes the artery locally or embolizes distally. Vulnerable plaques (those with a large lipid core, intraplaque hemorrhage, or a thin or ruptured fibrous cap) are prone to rupture and cause myocardial infarction due to coronary artery atherosclerosis. The second mechanism, unique …