Reproducibility of high-resolution MRI for the identification and the quantification of carotid atherosclerotic plaque components: consequences for prognosis studies and therapeutic trials.

BACKGROUND AND
PURPOSE: Although MRI is increasingly proposed to investigate composition of carotid atherosclerosis, its reproducibility has rarely been addressed. We assessed the reproducibility of MRI for the identification and quantification of carotid atherosclerotic plaque components.
METHODS: Using published criteria, 2 readers independently analyzed the carotid MRI (1.5-T MR units with a 4-channel phased-array surface coil, Machnet) of 85 consecutive patients with symptomatic (40% to 69% according to NASCET method) or asymptomatic (60% or greater) carotid artery stenosis enrolled in an ongoing prognostic study. One reader reevaluated all images. Fibrous cap was also secondarily identified independently on T2-weighted and time-of-flight (TOF) images.
RESULTS: Intraobserver agreement was substantial for the identification of calcifications (kappa [kappa]=0.70; 95% CI: 0.54 to 0.86) and lipid-rich/necrotic core (LR/NC) (kappa=0.69; 0.31 to 0.86), almost perfect for hemorrhages (kappa=0.82; 0.68 to 0.96), and moderate (kappa=0.58; 0.27 to 0.88) and fair (kappa=0.33; 0.09 to 0.56) for fibrous cap identification on T2-weighted and TOF images, respectively. Interobserver agreement was substantial for the identification of calcifications (kappa=0.74; 0.59 to 0.89) and hemorrhages (kappa=0.62; 0.43 to 0.81), and moderate for LR/NC (kappa=0.58; 0.20 to 0.95). Agreement was fair for fibrous cap identification on both T2-weighted (kappa=0.28; -0.03 to 0.59) and on TOF images (kappa=0.26; 0.04 to 0.48). Agreement between T2 and TOF images for fibrous cap identification was slight (kappa=0.16; 0.01 to 0.31). Intra- and interobserver reproducibility for quantitative area measurements of vessel, lumen, plaque, LR/NC, and fibrous components was high with intraclass correlation coefficients ranging from 0.73 to 0.99. However, for the LR/NC, the interval delimited by the Bland-Altman graphs was wide in comparison to the mean.
CONCLUSIONS: Vessel and plaque quantification is reproducible. Reproducibility of MRI for identifying and quantifying carotid plaque components is overall acceptable, but there is still significant variability that should be taken into account in the design of prognosis studies and clinical trials. Reproducibility for fibrous cap identification needs to be improved.