In-vivo quantitative T2 mapping of carotid arteries in atherosclerotic patients: segmentation and T2 measurement of plaque components

Background: Atherosclerotic plaques in carotid arteries can be characterized in-vivo by multicontrast cardiovascular magnetic resonance (CMR), which has been thoroughly validated with histology. However, the non-quantitative nature of multicontrast CMR and the need for extensive post-acquisition interpretation limit the widespread clinical application of in-vivo CMR plaque characterization. Quantitative T-2 mapping is a promising alternative since it can provide absolute physical measurements of plaque components that can be standardized among different CMR systems and widely adopted in multi-centre studies. The purpose of this study was to investigate the use of in-vivo T-2 mapping for atherosclerotic plaque characterization by performing American Heart Association (AHA) plaque type classification, segmenting carotid T-2 maps and measuring in-vivo T-2 values of plaque components. Methods: The carotid arteries of 15 atherosclerotic patients (11 males, 71 +/- 10 years) were imaged at 3 T using the conventional multicontrast protocol and Multiple-Spin-Echo (Multi-SE). T-2 maps of carotid arteries were generated by mono-exponential fitting to the series of images acquired by Multi-SE using nonlinear least-squares regression. Two reviewers independently classified carotid plaque types following the CMR-modified AHA scheme, one using multicontrast CMR and the other using T-2 maps and time-of-flight (TOF) angiography. A semi-automated method based on Bayes classifiers segmented the T-2 maps of carotid arteries into 4 classes: calcification, lipid-rich necrotic core (LRNC), fibrous tissue and recent IPH. Mean +/- SD of the T-2 values of vox Results: In 37 images of carotid arteries from 15 patients, AHA plaque type classified by multicontrast CMR and by T-2 maps (+ TOF) showed good agreement (76% of matching classifications and Cohen's kappa = 0.68). The T-2 maps of 14 normal arteries were used to measure T-2 of tunica intima and media (T-2 = 54 +/- 13 ms). From 11865 voxels in the T-2 maps of 15 arteries with advanced atherosclerosis, 2394 voxels were classified by the segmentation algorithm as LRNC (T-2 = 37 +/- 5 ms) and 7511 voxels as fibrous tissue (T-2 = 56 +/- 9 ms); 192 voxels were identified as calcification and one recent IPH (236 voxels, T-2 = 107 +/- 25 ms) was detected on T-2 maps and confirmed by multicontrast CMR. Conclusions: This carotid CMR study shows the potential of in-vivo T-2 mapping for atherosclerotic plaque characterization. Agreement between AHA plaque types classified by T-2 maps (+ TOF) and by conventional multicontrast CMR was good, and T-2 measured in-vivo in LRNC, fibrous tissue and recent IPH demonstrated the ability to discriminate plaque components on T-2 maps.