Arjen Lindenholz

55 The Use and Pitfalls of Intracranial Vessel Wall Imaging 3 Other necessary sequences Other, more ‘conventional’ MRI sequences also add helpful information when used in combination with intracranial vessel wall MRI. MR angiographic methods, such as contrast-enhancedMRA or time-of-flight (TOF-)MRA are particularly helpful to assess the arterial lumen and to identify the specific arteries to which the visualized vessel walls belong ( Figure 2 ). Also, in case of a small FOV, these techniques can guide FOV planning centered around the circle of Willis or focused on a specific (stenotic) vessel wall lesion. Contrast-enhanced MRA during first contrast passage clearly shows the arterial vasculature and is less sensitive to slow-flow artifacts compared with TOF- MRA. Furthermore, with strong elongation of the arterial vasculature it is also less sensitive to a signal decrease of cranial-caudal flow, e.g., in a strongly elongated MCA. Nonetheless, TOF-MRA is still a sensitive method to detect stenotic lesions and is the workhorse in most centers for the detection of vascular disease, including aneurysms of the intracranial vasculature. 52 Because it is not dependent on first contrast passage, acquisition time can be invested to increase the spatial resolution. Other pulse sequences that can be of additional value in vessel wall MRI assessment depend on the specific clinical question and include: (a) T 2 -weighted turbo spin- echo (TSE) sequence that can confirm the absence of a flow void in the arterial lumen in a patient with an arterial occlusion, (b) T 1 -weighted anatomic sequence for both assessment of normal anatomy and for use as ‘precontrast’ sequence for (mainly tissue) enhancement; (c) T 1 -weighted fat-suppressed sequence that can visualize a subintimal hematoma in patients with an arterial dissection that involves both the extracranial and intracranial segments, and (d) fluid-attenuated inversion recovery (FLAIR)- and DWI sequence for localizing white matter lesions and old and recent ischemia possibly associated with vessel wall pathology. The full vessel wall MRI protocol used in our institution, including key pulse sequence parameters, can be found in Table 1 and Table 2 . Figure 2. (A) 3T transverse Time-of-Flight (TOF) MR image (voxel size 0.4 x 0.7 x 1.0 mm 3 ) and (B) corresponding 3T transverse oblique precontrast T 1 -weighted vessel wall image (voxel size 0.5 x 0.5 x 1.0 mm 3 ), both zoomed in on the left P1/P2 bifurcation of the posterior cerebral artery (PCA; white arrow). On the vessel wall image (B) , the basal vein of Rosenthal can be seen coursing directly lateral to the PCA (arrowhead), which is not visible on the TOF-MRA.