Patients and clinical protocol

We retrospectively analyzed patients with AIS at our hospital from January 2012 to December 2014. Patients who showed acute major artery occlusion within 6 h from AIS onset were examined with MRI, including SWAN, DWI, and magnetic resonance angiography (MRA). We analyzed MRI at two stages: initial MRI (conducted at hospital admission) and follow-up MRI (conducted >24 h after AIS onset). Patients suffering from moyamoya disease or arterial dissection were excluded, as were patients in whom MRI or MRA assessments could not clearly determine acute recanalization within a few hours after initial treatment. The pathology in this study included occlusion of the internal carotid artery and or the M1 portion of the middle cerebral artery. Patients with other arterial occlusions were excluded.

To evaluate ischemic areas using SWAN and DWI, a modified Alberta Stroke Program Early CT Score (mASPECTS) was defined as follows: A total of 7 points were obtained for each of the M1, M2, M3, M4, M5, M6, and W portions of the vessel (Fig 1) [11, 12]. When the totals for the cortical and medullary veins on the occluded side were higher than on the normal side, we defined it as abnormal hypointensity in the SWAN evaluation. Hyperintensity for DWI was defined as abnormal intensity. When the occluded area showed abnormal intensity, a value of “1” was subtracted from 7 (a perfect mASPECTS). After calculating SWAN- and DWI-based mASPECTSs, we calculated the DWI-SWAN mismatch as subtraction of the initial SWAN-based mASPECTS from the initial DWI-based mASPECTS. We then calculated infarct growth by subtracting the follow-up DWI-based mASPECTS from the initial DWI-based mASPECTS. One neurosurgeon and one neuroradiologist independently assessed the mASPECTSs for DWI and SWAN from MRI scans. When their decisions were discordant, another neurosurgeon made the final decision. Each observer judged the MRI findings in each area, and the inter-rater agreement was calculated. We defined the arterial occlusive lesion (AOL) score of 2–3 as recanalization because collateral flow could affect the fate of the tissue in the recanalization group. The AOL score was defined using MRA source images or digital subtraction angiography. In patients without digital subtraction angiography, MRA was performed approximately 1 h after initial treatment to assess acute recanalization.

Case 1, cardioembolic stroke. (A) Initial diffusion-weighted imaging (DWI)-based modified Alberta Stroke Program Early CT Score (mASPECTS) was 6 points. (B) Initial T2*-weighted MR angiography (SWAN)-based mASPECTS was 0 points. (C) Magnetic resonance angiography (MRA) shows right M1 occlusion. (D) Case 2, atherothrombotic stroke. (E) Initial DWI-based mASPECTS was 6 points. (E) Initial SWAN-based mASPECTS was 5 points. (F) MRA shows right M1 occlusion.

A favorable outcome was defined as grades 0–2 by the modified Rankin Scale (mRS). The correlations between the DWI-SWAN mismatch and the final infarct lesion or the variables that affect clinical outcome were also evaluated.