2.2. Preoperative magnetic resonance imaging
MRI was performed using a 1.5-Tesla Vantage (Toshiba Medical Systems, Tokyo, Japan) or Avanto Scanner (Siemens, Erlangen, Germany) with a head and neck coil. Patient was placed in a standard dorsal position and the coil was combined with a head holder to stabilize position and prevent head rotation. A software syngo MR B19 2017 was used. A standardized MRI protocol was used to obtain four different contrast-weighted images (time-of-flight [TOF] and T1-, PD- and T2-wighted) of the carotid arteries 2 cm proximal and 2 cm distal to the carotid bifurcation in axial and coronary planes [22, 28]. Fat suppression was used to reduce signal from subcutaneous fat tissue. MR angiography of carotid arteries was performed with 3-dimensional TOF sequence (repetition time [TR]/time to echo [TE]/flip angle [FA] = 30 ms/8 ms/20°; matrix 160x256 mm).
Each carotid lesion was classified using the modified American Heart Association (AHA) classification of carotid plaques identified by MRI [28]. The Modified AHA classification was derived by Cai et al. from experience in reviewing MR images and extensive literature review [28]. In this classification, we used: I-II) near-normal wall thickness with no calcification (AHA classification types I and II were merged into the type I-II due to a lack of the current resolution of MRI), III) diffuse intimal thickening or small eccentric plaque with no calcification, IV-V) plaque with a lipid or necrotic core surrounded by fibrous tissue with possible calcification (for a similar reason, AHA classification types IV and V were merged into the type IV-V), VI) complex plaque with possible surface defect, haemorrhage or thrombus, VII) calcified plaque, and VIII) fibrotic plaque without lipid core and with possible small calcifications. According to this modified classification, types IV-V and VI were classified as unstable plaques, while the others were classified as stable plaques. MRI criteria for the diagnosis of plaque vulnerability were thin fibrous cap, large lipid-rich or necrotic core, and plaque disruption or intra-plaque haemorrhage. MR images were stored in a digital archive as DICOM files and independently analysed by two experienced investigators blinded to the clinical history and histological findings. Any discrepancies were resolved by discussion between the two investigators.