Ex-vivo imaging and plaque type classification of intracranial atherosclerotic plaque using high resolution MRI

Abstract

Background and Aims Recent development of high resolution MRI techniques have enabled imaging of intracranial atherosclerotic plaque in vivo. However, identifying plaque composition remains challenging given the small size and the lack of histological validation. This study aims to quantify the relaxation times of intracranial plaque components ex vivo at 3T and to determine whether multi-contrast MRI could classify intracranial plaque according to the American Heart Association classification with histological validation. Methods A total of 53 intracranial arteries with atherosclerotic plaques from 20 cadavers (11 male, age 73.8±10.9) were excised. Quantitative T1/T2/T2* mapping sequences and multi-contrast fast-spin echo sequences (T1, T2, proton-density weighted and short time inversion recovery) were acquired. Plaque components including: fibrous cap, lipid core, fibrous tissue, calcification, and healthy wall were segmented on histology, and their relaxation times were derived from quantitative images. Two radiologists independently classified plaque type blinded to the histology results. Results Relaxation times of plaque components are distinct and different. T2 and T2* values of lipid core are lower than fibrous cap (p=0.026 & p<0.0001), but are comparable with fibrous tissue and healthy wall (p=0.76 & p=0.42). MRI reliably classified plaque type compared with histology (κ=0.69) with an overall accuracy of 80.7%. The sensitivity and specificity using MRI to identify fibro-lipid atheroma (type IV–V) was 94.8% and 77.1%, respectively. Inter-observer agreement was excellent (κ=0.77). Conclusion Intracranial plaque components have distinct and different relaxation times at 3T. High-resolution MRI is able to characterize intracranial plaque composition and classify plaque types ex vivo at 3T.

The Use and Pitfalls of Intracranial Vessel Wall Imaging: How We Do It

“…Since no tissue can be obtained while the patient is alive-compared with, for example, endarterectomy samples in carotid artery disease-validation can only be performed in postmortem studies. A series of postmortem validation studies has been performed at 7 T by using ex vivo circle of Willis specimens from patients with and without a history of cerebrovascular disease, as well as from patients with intracranial aneurysms (47,(67)(68)(69)(70)(71). These studies found clear correlations between VW and atherosclerotic plaques detected on VW images and histopathologic findings, best seen on T1-weighted data and, thus, shorter imaging times.…”

“…reduce acquisition time in 3D TSE sequences, while the trajectories can also be optimized for reduced T2 weighting (17,23). Compressed sensing allows image reconstruction from fewer k-space sequences (47,67,68,70,71). However, more insight into what normal ageing of the VW looks like on VW images and what underlying mechanisms can cause VW (lesion) enhancement are still needed.…”

Identification and Quantitative Assessment of Different Components of Intracranial Atherosclerotic Plaque by Ex Vivo 3T High-Resolution Multicontrast MRI

“…The time between death and the postmortem examination was Ͻ1 month; however, the exact time window was unknown due to the limitations of our local ethics committee. The same specimens were evaluated in a previous study 18 ; however, the current study addresses distinctly different research questions, specifically performing a quantitative analysis of the identification of plaque components on multicontrast imaging. The demographics of the cadavers were reported in the previous publication.…”

“…The demographics of the cadavers were reported in the previous publication. 18 All specimens were rinsed carefully with saline to remove blood clots. The same pathologist excised possible atherosclerotic arteries according to gross morphologic changes.…”

“…Very few studies have reported intracranial plaque characteristics at 3T, the highest field strength that is clinically relevant. 14, 18 Thus, there is an urgent need to investigate whether clinical 3T scanners provide sufficient contrast for the evaluation of intracranial plaque composition. In addition, most of these 7T studies focused on the qualitative classification of plaque components and lacked quantitative assessment of highrisk plaque features, including the lipid core area, fibrous cap thickness, and plaque burden, all of which play an important role in atherosclerotic plaque progression and rupture.…”