Arjen Lindenholz

62 CHAPTER 3 Contrast enhancement Ideally, both pre- and postcontrast images are available for assessment of vessel wall (lesion) enhancement. With this setup contrast-enhancing lesions can be distinguished more accurately from non-enhancing lesions with a high signal intensity. To this end, we either compare pre- and postcontrast images one- on-one (eyeballing), or we calculate subtraction images (postcontrast minus precontrast), for example, using MeVisLab (Version 2.5, MeVis Medical Solutions AG, Bremen, Germany). However, as mentioned before, time constraints often limit vessel wall imaging in clinical practice to one postcontrast vessel wall MRI sequence. When only postcontrast images are available, assessment of contrast enhancement will be based on a comparison of the relative signal intensity of the arterial vessel wall (lesion) with the appearance of the other (contralateral) arterial vessel wall, the signal of the brain tissue and/or the pituitary stalk. The pituitary stalk shows vivid contrast enhancement, 57 and when the signal intensity of an intracranial arterial vessel wall lesion approximates that of the pituitary stalk, it can be considered as contrast enhancement ( Figure 4 ). Contrarily, if the signal intensity follows the intensity of the brain parenchyma, this can be considered as absence of enhancement. In healthy subjects, contrast enhancement of the internal carotid artery (ICA) and vertebral artery (VA) wall can be seen at the location where these arteries cross the dura mater, and should not be mistaken for pathological contrast enhancement ( Figure 5 ). Vasa vasorum and increased permeability of the endothelium have been described at this location, but the exact nature of this contrast enhancement has yet to be determined. 58 Also, the cavernous sinus shows diffuse enhancement after contrast administration; since the ICA runs through this cavernous venous plexus, it is difficult to assess the presence or absence of contrast enhancement of this cavernous segment of the internal carotid artery ( Figure 5 ).