A total of 396 consecutive acute ischemic stroke patients treated by endovascular means at our stroke unit, from March 2010 to May 2012, were screened. Inclusion criteria were isolated MCA M1 and/or M2 segment occlusion, pre-interventional MRI including SWI, and endovascular reperfusion therapy. A total of 88 patients fulfilled the inclusion criteria. The study was approved by our institutional review board. All patients gave written informed consent prior to enrolment in our stroke registry. If patients did not have the capacity to consent, written informed consent was obtained from their nearest relatives.
Clinical data on patients with acute ischemic stroke were prospectively recorded in our stroke database. Neurologic deficits were scored by a neurologist using the National Institutes of Health stroke scale (NIHSS). Clinical outcome was assessed 3 months after stroke using the modified Rankin scale (mRS).
Standard stroke MRI protocol was performed, which included diffusion-weighted imaging, T2-weighted imaging, time-of-flight magnetic resonance angiography (TOF-MRA), SWI, perfusion imaging, 3D first-pass gadolinium-enhanced MRA (GE-MRA) of the cervical and intracranial arteries, and T1-weighted post-contrast imaging. The images were acquired with a 1.5-T and 3-T MRI (Magnetom Avanto and Magnetom Verio; Siemens, Erlangen, Germany).
For the 1.5-T device, the SWI parameters were TR 49 ms, TE 40 ms, number of averages 1, FoV read 230 mm, FoV phase 75.0 %, voxel size 0.9 × 0.7 × 1.8 mm, flip angle 15°, acquisition time 2:59 min. For the 3-T device the parameters were as follows: TR 28 ms, TE 20 ms, number of averages 1, FoV read 230 mm, FoV phase 75.0 %, voxel size 0.9 × 0.9 × 2.0 mm, flip angle 15°, acquisition time 2:59 min. The SWI and minimum intensity projection (mIP) images were generated automatically by the scanner software.
The TOF-MRA parameters were as follows: 36 slices per slab, 4 slabs, 0.60 mm thickness, TR 22 ms, TE 3.6 ms, number of averages 1, FoV read 200 mm, FoV phase 90.6 %, voxel size 0.5 × 0.5 × 0.6 mm, flip angle 18°, acquisition time 5:16 min.
For the GE-MRA of the cranial and cervical arteries 88 coronal slices were acquired with 0.9 mm thickness, TR 3.21 ms, TE 1.2 ms, number of averages 1, FoV read 300 mm, FoV phase 100 %, voxel size 1.0 × 0.8 × 0.9 mm, flip angle 25°, 4 measurements, acquisition time 1:30 min.
DSA was performed via a transfemoral approach using a biplane, high-resolution angiographic system (Axiom Artis zee; Siemens, Erlangen, Germany).
Pre-interventional SWI and mIP series were analysed for the presence of a susceptibility vessel sign (SVS) in the MCA by three radiologists (C.W., R.K.V., A.K.). SVS was defined as an area of signal drop within the course of an artery that either exceeded the diameter of the contralateral non-occluded vessel, or that of the adjacent vessel segment on both the SWI and mIP series . Discrepancies between reviewers regarding thrombus visibility on SWI were resolved in consensus. After analysis of the SWI the presence of a vessel occlusion on TOF-MRA and GE-MRA corresponding to the proximal thrombus end on SWI was determined. The reviewers were aware of the presence of an MCA occlusion, but were blinded to lesion side and clinical symptoms.
Next, maximum thrombus length was measured on SWI using a previously published method . The location of the SVS was determined. The position of the proximal thrombus end was defined in relation to the midline (Figs. 1 and 2). The midline was defined as a line connecting the occipital part of the superior sagittal sinus with the point midway between the A2 segments of the anterior cerebral arteries as seen on the axial images. TOF-MRA, GE-MRA and diagnostic series of the DSA were analysed for the presence and location of the occluding thrombus according to the same criteria as on SWI. On the coronal GE-MRA (Fig. 2c) and the anteroposterior DSA projections (Fig. 2d), the midline was defined as a perpendicular line, midway between the A2 segments. Measurements were done by a radiologist in training (P.P.G.), who was aware of the presence of an SVS on SWI and the location of the occlusion on TOF-MRA, GE-MRA and DSA.
The following endovascular recanalization techniques were applied: stent retriever thrombectomy, thromboaspiration, extra- and intracranial stenting, intra-arterial thrombolysis or any combination of these. The interventional team selected material and thrombectomy techniques on the basis of the thrombus extension and angioarchitecture of the arteries proximal and distal to the occlusion site as seen on multimodal stroke MRI protocol. Stent retrievers were always deployed with the tip distal to the thrombus end. Cerebral reperfusion was assessed at the end of the endovascular intervention, on biplane or 3D rotational angiography, according to the thrombolysis in cerebral infarction (TICI) grading system .
Continuous variables are presented as mean ± standard deviation (SD) if not stated otherwise. Successful reperfusion was defined as TICI grades 2b–3. Pearson correlation coefficients were used to compare thrombus location (distance of proximal thrombus end to midline) between the different imaging modalities (i.e. SWI, TOF-MRA, GE-MRA and DSA). Bland–Altman plots were used to identify systematic differences between measurements of thrombus location on SWI and the other imaging modalities . Binary logistic regression was used to compare thrombus length on SWI with reperfusion success. The correlation between thrombus length on SWI and NIHSS score on admission was studied by Kendall’s τ rank correlation test. Associations between thrombus location and NIHSS score on admission were analysed with the Kruskal–Wallis test. A P value less than 0.05 was considered significant. Interobserver reliability for the detection of the SVS was studied using Cohen’s κ statistics. Statistics were performed with SPSS version 21 (IBM Corp., Armonk, NY, USA) and R version 3.0.3 (R Foundation for Statistical Computing, Vienna, Austria).