Subacute vessel wall imaging at 7-T MRI in post-thrombectomy stroke patients

Abstract

Purpose: Reports from 3-T vessel wall MRI imaging have shown contrast enhancement following thrombectomy for acute stroke, suggesting potential intimal damage. Comparisons have shown higher SNR and more lesions detected by vessel wall imaging when using 7 T compared with 3 T. The aim of this study was to investigate the vessel walls after stent retriever thrombectomy using high-resolution vessel wall imaging at 7 T.

Methods: Seven patients with acute stroke caused by occlusion of the distal internal carotid artery (T-occlusion), or proximal medial cerebral artery, and treated by stent retriever thrombectomy with complete recanalization were included and examined by 7-T MRI within 2 days. The MRI protocol included a high-resolution black blood sequence with prospective motion correction (iMOCO), acquired before and after contrast injection. Flow measurements were performed in the treated and untreated M1 segments.

Results: All subjects completed the MRI examination. Image quality was independently rated as excellent by two neuroradiologists for all cases, and the level of motion artifacts did not impair diagnostic quality, despite severe motion in some cases. Contrast enhancement correlated with the deployment location of the stent retrievers. Flow data showed complete restoration of flow after treatment.

Conclusion: Vessel wall imaging with prospective motion correction can be performed in patients following thrombectomy with excellent imaging quality at 7 T. We show that vessel wall contrast enhancement is the normal post-operative state and corresponds to the deployment location of the stent retriever.

Keywords: 7-T MRI; Embolic stroke; Stent retriever; Thrombectomy; Vessel wall imaging.

Fig. 1

Transaxial images post-Gd images from all 7 subjects showing the Gd uptake in the vessel walls at a location corresponding to the position of the stent retriever (arrows in left and middle columns), with absence of Gd uptake on the contralateral side (right column). The asterisk indicates a basal ganglia infarct in patient 6. All images in the left column were reconstructed from sagittal acquisitions with meticulous adjustments to obtain exact transaxial reconstructions to allow comparison with the contralateral side. The images in the middle and right columns were reconstructed to obtain image planes perpendicular to the center-line to illustrate the circumferential distribution of vessel wall edema pre-Gd (middle column Gd−) and the post-Gd enhancement (middle column Gd+). On the contralateral side (right column), the vessel vas often barely visible, and in cases where the vessel was occasionally thicker, suggesting intracranial atherosclerosis, little or no Gd-uptake was seen (illustrated in patient 3/control Gd− and Gd+)

Fig. 2

a Illustration of the typical findings in one of the patients. The site of the embolus was determined from the dens artery sign on non-enhanced CT (white arrow), b occlusion on CTA (white arrows indicating the proximal and distal end of the embolus), and c DSA images from the thrombectomy procedure (white arrows indicating the proximal and distal end of the embolus, white arrowhead indicating the distal end of the stent retriever). The dominant M2 branch distal of the embolus in which the stent retriever was deployed (the white arrow in panel D indicates the distal end of the embolus, the white arrowhead in panel E indicating the distal end of the stent retriever) shows vessel wall Gd uptake corresponding to the entire length of the stent retriever (white arrowheads in panel F). d–f The black arrows illustrate another M2 branch distal of the occlusion that shows no vessel wall Gd uptake

Fig. 3

Effects of prospective motion correction. The monitored motion during the post-Gd acquisition for the patient that moved the most (patient 7) (a). The translations (top left panel) shows the translation in millimeters in x, y and z rotations (middle left panel) shows the position relative to the start in degrees. During the scan, the motion score (bottom left panel) is continuously monitored. In this case, the patient moved almost 1 cm, and rotated up to 6°. Motion larger than the chosen threshold (dashed line in the lower left panel, set to 1 mm in our protocol) triggers reacquisition of the previously acquired k-space segment. Despite this severe motion, the image quality was excellent as shown by an image from the same acquisition (b). The percentage of reacquired data for all 14 vessel wall scans, clearly showing that the movements in patient 6 and 7 demanded reacquisition of a large part of the data (c)

Fig. 4

Measurements in the MCA M1 branch on the treated and contralateral side (N = 4). a The placement of the flow measurement planes, b the net flow volume (ml/heart beat), c the average velocity (cm/s), and d total flow (L/min). There is no significant difference in the average values for any variable, but in this small sample there is a trend towards a larger spread of values in the treated vessel