Intracranial Vessel Wall MRI: An Emerging Technique With a Multitude of Uses

Magnetic resonance (MR) imaging of the intracranial arterial wall differs from conventional angiographic techniques (computed tomography angiography (CTA), magnetic resonance angiography (MRA), and catheter angiography), which depict the lumen. The ability to image the arterial wall is useful, as many pathological processes reside within the wall and only secondarily affect the lumen. In a very limited sense, intracranial vessel wall MRI (VW-MRI) has been around since the early years of MRI. For example, even routine pulse sequences will show the markedly thickened vessel wall and intramural hematoma in patients with so-called giant intracranial aneurysms. However, the term intracranial VW-MRI is usually used in reference to imaging that is optimized and targeted for evaluation of much smaller vessel wall abnormalities. This imaging relies on MRI pulse sequences that null the signal in the blood and cerebrospinal fluid adjacent to the vessel wall, making it possible to visualize the wall even though it is thin compared with voxel dimensions. In this review, we will briefly discuss the technical aspects of intracranial VW-MRI, and then present 6 emerging clinical uses for VW-MRI to highlight the versatility of the technique. IMAGING TECHNIQUE MRI pulse sequences optimized for routine brain imaging are typically not useable, in their existing forms, for intracranial VW-MRI. However, it is often possible to modify the scan parameters of commercially available sequences and obtain images that adequately depict the vessel wall. The most important technical requirements for VW-MRI are high spatial resolution, and nulling of signal in intravascular blood and cerebrospinal fluid, to improve visualization of the vessel wall. The specific spatial resolution required depends on the disease one is studying, but typical in-plane voxel dimensions are in the order of 0.5 mm, and typical slice thickness is in the order of 0.5 mm if using a 3-dimensional pulse sequence, or 2 mm...