Arjen Lindenholz

162 CHAPTER 7 segments that were analyzed included the left and right intracranial internal carotid artery (ICA, the clinoid, supra-clinoid and terminal segments) and the left and right middle cerebral artery (MCA, M1 and M2 segments). Recanalization after IAT was assessed with TICI-grading on post-procedural DSA images. 19 First, in the IAT-group post-procedural DSA, 7T MR vessel wall, and 7T MRA images were assessed for dissections and stenoses as major vessel wall changes. Intracranial stenoses were classified into < 50% stenosis (minor), 50-69% stenosis (moderate), 70-99% stenosis (severe), and occlusion. 20 Second, all 7T MR vessel wall images were assessed for the presence and number of enhancing foci per arterial segment. The mean numbers of enhancing foci of both readers were used for the analyses. Assessment was blinded to IAT and non-IAT. In all patients (IAT-group and non-IAT group) the arteries in the hemisphere ipsilateral to the ischemic infarction were compared with those of the contralateral side. All enhancing foci were further classified as either concentric (circumferential of the vessel wall > 50% enhancing) or eccentric ( < 50% circumference enhancement) type of enhancement. Intracranial atherosclerosis more often shows eccentric vessel wall enhancement and an inflammatory state of the vessel wall show most often concentric vessel wall enhancement. 21,22 The method and cut-off point for the configuration of vessel wall lesion assessment by visual inspection has been described before as a clinically usable tool for vessel wall assessment. 21 {Lindenholz, 2018 #3944} In the assessment of contrast enhancement, a focus was considered as enhancing when the signal intensity approximated the signal intensity of the (enhancing) pituitary stalk and was present in at least two slices. Next, pre- and postcontrast vessel wall images were compared side-by-side to confirm the enhancement. As a double confirmation of enhancement subtraction images were calculated and used. For this purpose, pre- and postcontrast vessel wall images were coregistered for the whole 3D volume using the elastix toolbox in MeVisLab (version 2.7; MeVis Medical Solutions, Bremen, Germany). 23 Subsequently, precontrast vessel wall images were subtracted from the co-registered postcontrast vessel wall images and assessed for contrast enhancement. The registration parameters, Δ Rotation (in degrees) and Δ Translation (in millimeters), were used as a measure of motion between pre- and postcontrast vessel wall sequences. The Δ Rotation and Δ Translation parameters were calculated as √ (X-axis 2 + Y-axis 2 + Z-axis 2 ). When more than one enhancing focus was detected within one arterial segment, they were counted separately when they were separated from each other by a normal appearing vessel wall segment in between. Also, enhancing foci at the location where the ICA crosses the dura mater from extracranial to intracranial, suspicious for vasa vasorum, were not considered as vessel wall enhancement.