Arjen Lindenholz

71 The Use and Pitfalls of Intracranial Vessel Wall Imaging 3 In current protocols, several scan parameters can be varied to counterbalance the relatively long acquisition time, such as the oversample factor, TSE train length, and image acceleration by parallel imaging such as SENSE that leads to a change in imaging time. 1,21,31,40 However, all of these techniques sacrifice spatial resolution and/or SNR in the process, or need more advanced hardware (e.g., a higher number of receiver coils or multi-band scanning) to overcome the inevitable cost in image quality. Clinical considerations Because no gold standard in vivo method for intracranial arterial vessel wall pathology is available, histologic validation of intracranial vessel wall MRI is essential. Since no tissue can be obtained while the patient is alive – compared with, for example, endarterectomy samples in carotid artery disease – validation can only be performed in postmortem studies. A series of postmortem validation studies has been performed at 7T by using ex vivo circle of Willis specimens from patients with and without a history of cerebrovascular disease, as well as from patients with intracranial aneurysms. 47,67-71 These studies found clear correlations between vessel wall and atherosclerotic plaques detected on vessel wall MRI and histopathological findings, best seen on the T 1 -weighted sequences. 47,67,68,70,71 However, more insight into what normal ageing of the vessel wall looks like on vessel wall MRI, and what underlying mechanisms can cause vessel wall (lesion) enhancement are still needed. This has proven to be a challenge, becausemethods to preserve tissue in ex vivo studies (e.g., fixation and tissue temperature effects) change the MRI characteristics of the tissue so that results from these studies cannot be directly translated to in vivo vessel wall MRI, while functional measures, such as lesion enhancement after contrast administration, cannot be performed in postmortem studies. Conclusion Intracranial vessel wall MRI has become part of state-of-the-art MRI protocols detecting causes of ischemic stroke, mainly in a research setting but increasingly askedfor (andused) inclinical practice. Ithas tentativelyshowncommonlyseenvessel wall changes in patients with diseases including, but not limited to, central nervous system vasculitis, moyamoya disease, aneurysms, dissections and intracranial atherosclerosis. However, its precise role and added value for prognosis and patient care needs further elucidation. A field strength of at least 3T enables vessel wall MRI sequenceswithhighenoughCNRandspatial resolution toassess the thin intracranial atherosclerotic vessel wall and detect and ultimately characterize vessel wall lesions. Radiologists should be aware of the normal appearance (variance) of the vessel wall on intracranial vessel wall MRI, the main characteristics of vessel wall lesions that