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

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 T₂ 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 T₂ mapping for atherosclerotic plaque characterization by performing American Heart Association (AHA) plaque type classification, segmenting carotid T₂ maps and measuring in-vivo T₂ values of plaque components.

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). T₂ 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 T₂ maps and time-of-flight (TOF) angiography. A semi-automated method based on Bayes classifiers segmented the T₂ maps of carotid arteries into 4 classes: calcification, lipid-rich necrotic core (LRNC), fibrous tissue and recent IPH. Mean ± SD of the T₂ values of voxels classified as LRNC, fibrous tissue and recent IPH were calculated.

In 37 images of carotid arteries from 15 patients, AHA plaque type classified by multicontrast CMR and by T₂ maps (+ TOF) showed good agreement (76% of matching classifications and Cohen’s κ = 0.68). The T₂ maps of 14 normal arteries were used to measure T₂ of tunica intima and media (T₂ = 54 ± 13 ms). From 11865 voxels in the T₂ maps of 15 arteries with advanced atherosclerosis, 2394 voxels were classified by the segmentation algorithm as LRNC (T₂ = 37 ± 5 ms) and 7511 voxels as fibrous tissue (T₂ = 56 ± 9 ms); 192 voxels were identified as calcification and one recent IPH (236 voxels, T₂ = 107 ± 25 ms) was detected on T₂ maps and confirmed by multicontrast CMR.

This carotid CMR study shows the potential of in-vivo T₂ mapping for atherosclerotic plaque characterization. Agreement between AHA plaque types classified by T₂ maps (+TOF) and by conventional multicontrast CMR was good, and T₂ measured in-vivo in LRNC, fibrous tissue and recent IPH demonstrated the ability to discriminate plaque components on T₂ maps.