Arjen Lindenholz

65 The Use and Pitfalls of Intracranial Vessel Wall Imaging 3 the hyperintense signal: Slow-flow artifacts are often symmetrically present in both hemispheres and can be seen in many vascular structures both inside the skull and in the extracranial veins, such as the superficial temporal vein ( Figure 6 ). Suboptimal suppression of slow flow is related to the vessel wall MRI technique used, and although TOF-MRA is also prone to slow-flow artifacts, it rarely occurs that these artifacts occur at the exact same location at both sequences. When in doubt about whether a vessel wall abnormality is a slow-flow artifact or a real vessel wall lesion, we do not want to judge every abnormality as a vessel wall lesion, but we analyze every possible lesion with caution; when in doubt, we may not call it a vessel wall lesion; when the hyperintense signal is too large in volume for a thin arterial vessel wall, we consider it a slow-flow artifact. The other artifact that can cause difficulties in interpretation of vessel wall abnormalities is the free induction decay (FID) artifact ( Figure 7 ). This artifact is caused by repeated refocusing radiofrequency pulses within the short echo times of T 1 -weigthed vessel wall MRI sequences. The artifact occurs when the free induction decays from the refocusing pulses are not completely spoiled, and appears as a linear dashed pattern in the image. 59 The vessel wall will show the same dashed, ‘zigzag’ pattern, making it difficult to differentiate between artifact and vessel wall thickening and/or lesion. Possible ways to minimize this artifact is to increase the number of signals acquired, increase the echo time or choose a larger slice thickness; however, for obvious reasons these are difficult to implement in intracranial vessel wall MRI. Vessel wall disease Although intracranial vessel wall MRI is still in its developmental stage, there are already several challenging diagnostic situations in which it has shown its potential value, 1,2,60,61 the most important of which is determining the cause of stroke and assessing accompanying vessel wall lesions. Recently, two excellent review papers have been published that both address the commonly found imaging characteristics of different types of intracranial vessel wall pathology, including a thorough literature overview. 2,62 We will therefore restrict this section to a compact overview table ( Table 3 ) for hands-on use in clinical practice, including illustrative image examples ( Figure 4 , Figure 8 , Figure 9 , Figure 10 , Figure 11 and Figure 12 ).