In-vivo quantitative T(2) mapping of carotid arteries in atherosclerotic patients: segmentation and T(2) 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 voxels classified as LRNC, fibrous tissue and recent IPH were calculated. 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 κ = 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.