This retrospective cross-sectional study was approved by our institutional review board and all methods were performed in compliance with the Helsinki Declaration. The need for informed consent was waived by Shahid Beheshti Medical University ethics committee (IR.SBMU.MSP.REC.1398.420). All consecutive patients who underwent brain MRI and MRA to evaluate for suspected cerebrovascular accident at Loghman Hakim hospital between March 20, 2017, and April 10, 2018, were included in this study. Those with a history of head trauma, craniotomy or craniectomy, vasculitis, pregnancy, vascular malformation (including aneurysm or arteriovenous malformation), hemorrhagic infarction, significant stenosis or occlusion of internal carotid artery, significant stenosis or occlusion of basilar artery or its major branches were excluded from the study. Demographic data, patient’s presenting symptoms, and past medical history including hypertension, diabetes, hyperlipidemia, heart disease, and smoking were reviewed from the patients’ electronic medical records.
Scans were performed using a 1.5-T Vantage Elan magnet (Canon Medical Systems, Otawara, Tochigi, Japan). The examination was performed without intravenous contrast and included echo-planar T1-weighted images (TR: 591 ms, TE:15 ms, Spatial Resolution: 6.2 mm, FoV: 230 mm*230 mm), T2-weighted images (TR: 4048 ms, TE:90 ms, Spatial Resolution:6.2 mm, FoV: 230 mm*230 mm), FLAIR, and diffusion-weighted images (DWI).
For evaluation of circle of Willis, images were obtained in 3 slabs each containing 30 slides with 3-dimensional time-of-flight MRA technique (TR: 22 ms, TE:7 ms, number of acquisitions: 2; flip angle: 17°; 1 mm slice thickness with a 0.5 mm overlap; matrix size: 256*160, FOV: 200*200 mm). MRA images were reconstructed in transverse oblique planes using a maximum intensity projection algorithm.
All images were reviewed by a national board-certified radiologist with 5 years of experience in neuroimaging (M.H.) and a fourth-year radiology resident (E.S.) using a consensus approach.
The following variations in posterior aspect of the circle of Willis on MRA were recorded: Partial or complete fetal origin of posterior cerebral artery (pfPCA and cfPCA) and aplasia or hypoplasia of PCoA.
CfPCA was defined as the P1 segment being absent and PCA originating completely from the internal carotid artery (ICA). PfPCA was defined as the existing P1 segment with a caliber equal to or smaller than PCoA .
Due to the limited resolution of MRA, it was difficult to differentiate PCoA hypoplasia (< 1 mm in diameter) from aplasia (absence of PCoA). Therefore, we considered hypoplasia/aplasia together, defined as PCoA diameter of < 1 mm or non-visualization of PCoA .
In cases of infarction, the affected side, vascular territory, and the age of infarct (acute/subacute versus chronic) were recorded based on the MRI findings and clinical history. The areas of restricted diffusion (bright signal on DWI and corresponding low ADC values) were qualitatively determined based upon consensus by the reviewing radiologists and were considered acute/subacute infarct. The areas of encephalomalacia or gliosis without associated restricted diffusion and with volume loss were considered chronic infarct. In cases with simultaneous acute and chronic infarcts, the territory with acute infarct was considered.
The vascular distribution of infarct was divided into anterior, posterior, thalamus, and watershed area similar to previous studies .
In the presence of unilateral anatomical variation, if it was on the same side as the infarct, it was considered positive, so we described it as “ipsilateral anatomic variation.” When the patient had bilateral anatomical variation, the side with infarction was counted and reclassified as “ipsilateral.”
The results were presented as mean ± standard deviation (SD) for quantitative variables and were summarized by absolute frequencies and percentages for categorical variables. Categorical variables were compared using Chi-square test or Fisher's exact test when more than 20% of cells with expected count of less than 5 were observed. Quantitative variables were compared using the student T test and Mann Whitney U test, for parametric and nonparametric variables, respectively. For the statistical analysis, the statistical software SPSS version 21.0 for windows (SPSS Inc., Chicago, IL) was used. p values of 0.05 or less were considered statistically significant.