Arjen Lindenholz

59 The Use and Pitfalls of Intracranial Vessel Wall Imaging 3 Figure 3. (A) 3T oblique (transverse/coronal) precontrast T 1 -weighted vessel wall image (voxel size 0.5 x 0.5 x 1.0 mm 3 ) of a 29-year-old healthy volunteer. In this single slice, most large intracranial arteries of the circle of Willis are (partially) visible, e.g., the left A1 segment of the anterior cerebral artery (ACA) and basilar artery (arrows). (B) Zoomed image of the M1 segment of the left middle cerebral artery (MCA). The vessel wall (white arrow) can barely be seen because of its close proximity to the brain parenchyma which has comparable signal intensity. (C) Reconstructed sagittal image (0.6 x 1.0 in-plane spatial resolution) of the left MCA (white arrow). The quality and detail are marginally less compared with the transverse/ oblique image because of the anisotropic voxel size. In general a vessel wall lesion is definedby one or bothof the following characteristics ( Figure 4 ): (a) a focal or more diffuse thickening of the vessel wall greater than 50% compared with the adjacent vessel wall thickness; 31 and/or (b) focal or diffuse vivid contrast enhancement. 56 One can further characterize the lesion as eccentric – less than 50% of the circumference of the vessel wall – or concentric – greater than 50% of the circumference – and as enhancing or not enhancing after contrast administration (see below), which can give an indication of the specific underlying disease process ( Table 3 ). When assessing a potential lesion, the area of interest should be compared with next sections (in case of concentric thickening) or the cross-section of the vessel wall should be scrutinized (in case of eccentric thickening), and it should be compared with the contralateral vessel wall (but beware of bilateral disease). Finally, vessel wall images should be cross-correlated with the axial TOF- MRA images for correct interpretation of the specific artery/arteries in which the lesion(s) was/were found, and to check if the lesion causes luminal narrowing. Bear in mind that due to arterial remodeling, vessel wall lesions very often do not show a stenosis; 15 therefore, the absence of stenosis should not be an argument in the decision process whether a lesion is present or not.