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.