In-vivo T2 mapping of atherosclerotic plaques in carotid arteries
Journal of Cardiovascular Magnetic Resonance volume 14, Article number: P134 (2012)
The purpose of this study was to measure the T2 relaxation times of carotid atherosclerotic plaque components in-vivo at 3T and show the potential application of T2 mapping for plaque segmentation.
Clinical studies that have measured plaque T2 times were mostly performed ex-vivo using a small number of plaques excised from different arterial locations and imaged at different field strength using a limited number of TEs. The Table shows that T2 measured in the lipid-rich necrotic core (LRNC) was consistently shorter than T2 in fibrous tissue or in normal media, which showed similar values. The only two studies of carotid plaques showed a comparable T2 range for LRNC and fibrous tissue regardless of field strength difference.
12 patients with stable atherosclerosis (9 males, 72±11 years) were imaged on a 3T scanner (Siemens TIM Trio). Ethics approval from local board was obtained and subjects gave informed consent. A multiple-Spin-Echo (multi-SE) sequence (Spin-Echo_Multi-Contrast or SE_MC) with low SAR pulses acquired black-blood cross-sectional images of carotid arteries using a 4-channel surface coil and cardiac gating (TE=25.8-38.7-51.6-64.5-77.4-90.3-103.2ms, TR=2R-R, FOV=160×128mm2, matrix-size=320×256, slice-thickness=2mm, partial-Fourier=5/8). T2 was estimated for every voxel of the carotid wall by fitting a mono-exponential decay curve to the signal intensities at 7 TEs using non-linear least-squares regression. Using a semi-automated method based on Bayes classifiers, T2 maps of carotid arteries were segmented in 4 tissue types: calcification; LRNC; fibrous tissue and normal media; intra-plaque haemorrhage. Histological validation was not available. AHA plaque classification was performed by two blinded reviewers on multi-contrast images acquired separately.
23 carotid arteries presented visible lesions graded using the MRI-modified AHA scheme: 10 type III, 7 type IV-V, 2 type VI, 2 type VII and 2 type VIII plaques. From the T2 map segmentation of these arteries (Figure 1), 3438 voxels were classified as LRNC with T2=36±5ms and 10291 voxels as fibrous tissue or normal media with T2=55±9ms (Table 1). Due to low proton density, calcification produced insufficient SNR and T2 could not be measured. 1212 voxels were classified as haemorrhage with T2=89±20ms (some were incorrectly included due to T2 overestimation).
This study showed the potential of in-vivo T2 mapping for atherosclerotic plaque characterization using Multi-SE in carotid imaging. T2 relaxation times measured in-vivo for LRNC and fibrous tissue or media at 3T were consistent with the range of values reported in literature for carotid plaques.
This study was funded by EPSRC.
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Biasiolli, L., Lindsay, A.C., Choudhury, R.P. et al. In-vivo T2 mapping of atherosclerotic plaques in carotid arteries. J Cardiovasc Magn Reson 14 (Suppl 1), P134 (2012). https://doi.org/10.1186/1532-429X-14-S1-P134