Ex vivo differential phase contrast and magnetic resonance imaging for characterization of human carotid atherosclerotic plaques

Imaging the microstructure of the arterial wall - ex vivo to in vivo potential

Microstructural imaging enables researchers to visualise changes in the arterial wall, allowing for (i) a deeper understanding of the role of specific components in arterial mechanics, (ii) the observation of cellular responses, (iii) insights into pathological alterations in tissue microstructure, and/or (iv) advancements in tissue engineering aimed at replicating healthy native tissue. In this prospective review, we present various imaging modalities spanning from ex vivo to in vivo applications within arterial tissue. The pros, cons, and sensitivities of these modalities are highlighted. By consolidating the latest advancements in microstructural imaging of arterial tissue, the authors aim for this paper to serve as a guide for researchers designing experiments at various stages. Furthermore, the integration of non-invasive, non-destructive imaging techniques into studies provides an additional layer of microstructural information, enhancing scientific findings, improving our understanding of disease, and potentially enabling earlier or more effective diagnostic capabilities. STATEMENT OF SIGNIFICANCE: Imaging the specific microstructural components of the arterial wall provides critical insights into vascular biology, mechanics, and pathology. It enables the visualisation of key structural components and their roles in arterial function, supports the analysis of cell-matrix interactions, and reveals microarchitectural changes associated with disease progression. This level of specificity also informs the design of biomimetic materials and scaffolds in tissue engineering, facilitating the replication of native arterial properties. By synthesising recent developments in microstructural imaging techniques, this paper serves as a reference for investigators designing experiments across a range of vascular research applications. Moreover, the incorporation of non-invasive, non-destructive imaging methods offers a means to acquire detailed microstructural data without compromising tissue integrity. This enhances the interpretability and translational potential of findings, deepens our understanding of vascular disease mechanisms, and may ultimately contribute to the development of earlier and more precise diagnostic approaches.

Microstructural and mechanical insight into atherosclerotic plaques: an ex vivo DTI study to better assess plaque vulnerability

Non-invasive microstructural characterisation has the potential to determine the stability, or lack thereof, of atherosclerotic plaques and ultimately aid in better assessing plaques' risk to rupture. If linked with mechanical characterisation using a clinically relevant imaging technique, mechanically sensitive rupture risk indicators could be possible. This study aims to provide this link-between a clinically relevant imaging technique and mechanical characterisation within human atherosclerotic plaques. Ex vivo diffusion tensor imaging, mechanical testing, and histological analysis were carried out on human carotid atherosclerotic plaques. DTI-derived tractography was found to yield significant mechanical insight into the mechanical properties of more stable and more vulnerable microstructures. Coupled with insights from digital image correlation and histology, specific failure characteristics of different microstructural arrangements furthered this finding. More circumferentially uniform microstructures failed at higher stresses and strains when compared to samples which had multiple microstructures, like those seen in a plaque cap. The novel findings in this study motivate diagnostic measures which use non-invasive characterisation of the underlying microstructure of plaques to determine their vulnerability to rupture.

Sub-micrometer morphology of human atherosclerotic plaque revealed by synchrotron radiation-based μCT—A comparison with histology

Histology is a long standing and well-established gold standard for pathological characterizations. In recent years however, synchrotron radiation-based micro-computed tomography (SRμCT) has become a tool for extending the imaging of two-dimensional thin sections into three-dimensional imaging of tissue blocks, enabling so-called virtual histology with arbitrary clipping planes, volumetric rendering and automatic segmentation. In this study, we present a thorough characterization of human carotid plaques after endarterectomy of patients with stroke or transient ischemic attack (TIA), investigating several different pathologic structures using both SRμCT and histology. Phase-contrast SRμCT was performed with two different magnifications (voxel sizes 6.5 μm and 0.65 μm, respectively), and histology was performed with multiple different stainings (Alpha-actin, Glycophorin A, von Kossa, Movat, CD68). The 0.65 μm high-resolution SRμCT was performed on selected areas with plaque typical relevant morphology, identified on the 6.5 μm low-resolution SRμCT. The tomography datasets were reconstructed with additional 3D volume rendering and compared to histology. In total, nine different regions with typical pathologic structures were identified and imaged with high-resolution SRμCT. The results show many characteristics typical for advanced atherosclerotic plaques, clinically relevant, namely ruptures with thrombosis, neo-vascularization, inflammatory infiltrates in shoulder regions, lipid rich necrotic cores (LRNC), thin fibrous cap, calcifications, lumen irregularities, and changes in vessel wall structures such as the internal elastic membrane. This method’s non-destructive nature renders details of micro-structures with an excellent visual likeness to histology, with the additional strength of multiplanar and 3D visualization and the possibility of multiple re-scans.

Intraplaque Microvascular Flow Signal in Superb Microvascular Imaging and Magnetic Resonance Imaging Carotid Plaque Imaging in Patients with Atheromatous Carotid Artery Stenosis

Carotid artery atherosclerosis is one of the major risk factors for ischemic stroke. Intraplaque neovascularization (IPN) is one of the steps toward the development of vulnerable plaque. Superb microvascular imaging (SMI) is a new ultrasonographic technique for visualizing low-velocity and microvascular flow by clutter suppression to extract flow signals from large to small vessels and enables visualization of intraplaque microvascular flow (IMVF) without echo contrast media. We aimed to investigate the association between IMVF signal in SMI and MRI plaque imaging among patients with atherosclerotic carotid stenosis. We prospectively enrolled patients (>18 years old) with mild to severe carotid stenosis (more than 50% in cross-sectional area) diagnosed by carotid ultrasonography between August 2017 and April 2018, irrespective of sex and history of stroke. A total of 40 patients (31 men, 9 women; mean age, 75.1 ± 10.0 years) were enrolled. SMI revealed IPN findings in 21 patients. SMI clearly visualized the direction of pulsatile flow movement in microvessels and IPN was easily classified into the two types of Type V (n=2) and Type E (n=19). Multivariate logistic regression analysis presented that microvascular flow signal in carotid plaque on SMI was identified as a significant predictor of intraplaque hemorrhage as evaluated by MRI (OR, 8.46; 95%CI, 1.44-49.9; p=0.018). This study demonstrated a significant association between the presence of IMVF signal in SMI and intraplaque hemorrhage characterized by high-intensity lesions on MRI T1-FFE images.

Quantitative Analysis of Lipid-Rich Necrotic Core in Carotid Atherosclerotic Plaques by In Vivo Magnetic Resonance Imaging and Clinical Outcomes

Background

The aim of this study was to explore the accuracy of in vivo magnetic resonance imaging (MRI) in the quantitative evaluation of lipid-rich necrotic core (LRNC) in carotid atherosclerotic plaques compared with histopathology, and to assess the association of LRNC size with cerebral ischemia symptoms.

Material/Methods

Thirty patients were enrolled and 19 patients (16 men and 3 women) were analyzed. All the patients were submitted to MRI on a Siemens Avanto (1.5-Tesla) device before carotid endarterectomy (CEA). The scanning protocol included three-dimensional time of flight (3D TOF), T₁-weighted image (T₁WI), T₂-weighted image (T₂WI), turbo spin-echo T₂-weighted (T₂-TSE), and contrast-enhanced T₁-weighted image. MRI images were reviewed for quantitative measurements of LRNC areas. LRNC specimens were collected for histology. Percentages of LRNC area to total vessel area were assessed to determine the association of MRI with histological findings.

Results

There were 151 pairs of matched MRI and pathological sections. LRNC area percentages (LRNC area/vessel area) measured by MRI and histology were 20.6±9.0% and 18.7±9.5%, respectively (r=0.69, p<0.001). Twelve out of 19 patients had symptoms (S-group; 3 had recent stroke, 3 had a recent stroke and a history of transient ischemic attack (TIA), and 6 had TIA); the remaining 7 subjects showed no symptoms (NS-group). LRNC area percentages in the S- and NS-groups were 22.2±5.8% and 12.6±10.7%, respectively (p<0.05).

Conclusions

MRI can quantitatively measure LRNC in carotid atherosclerotic plaques, and may be useful in predicting the rupture risk of plaques. These findings provide a basis for imaging use in individualized treatment plan.

Functional assessment of the stenotic carotid artery by CFD-based pressure gradient evaluation

The functional assessment of a hemodynamic significant stenosis base on blood pressure variation has been applied for evaluation of the myocardial ischemic event. This functional assessment shows great potential for improving the accuracy of the classification of the severity of carotid stenosis. To explore the value of grading the stenosis using a pressure gradient (PG)-we had reconstructed patient-specific carotid geometries based on MRI images-computational fluid dynamics were performed to analyze the PG in their stenotic arteries. Doppler ultrasound image data and the corresponding MRI image data of 19 patients with carotid stenosis were collected. Based on these, 31 stenotic carotid arterial geometries were reconstructed. A combinatorial boundary condition method was implemented for steady-state computer fluid dynamics simulations. Anatomic parameters, including tortuosity (T), the angle of bifurcation, and the cross-sectional area of the remaining lumen, were collected to investigate the effect on the pressure distribution. The PG is highly correlated with the severe stenosis (r = 0.902), whereas generally, the T and the angle of the bifurcation negatively correlate to the pressure drop of the internal carotid artery stenosis. The calculation required <10 min/case, which made it prepared for the fast diagnosis of the severe stenosis. According to the results, we had proposed a potential threshold value for distinguishing severe stenosis from mild-moderate stenosis (PG = 0.88). In conclusion, the PG could serve as the additional factor for improving the accuracy of grading the severity of the stenosis.

Advanced MRI for carotid plaque imaging

Atherosclerosis is the ubiquitous underling pathological process that manifests in heart attack and stroke, cumulating in the death of one in three North American adults. High-resolution magnetic resonance imaging (MRI) is able to delineate atherosclerotic plaque components and total plaque burden within the carotid arteries. Using dedicated hardware, high resolution images can be obtained. Combining pre- and post-contrast T1, T2, proton-density, and magnetization-prepared rapid acquisition gradient echo weighted fat-saturation imaging, plaque components can be defined. Post-processing software allows for semi- and fully automated quantitative analysis. Imaging correlation with surgical specimens suggests that this technique accurately differentiates plaque features. Total plaque burden and specific plaque components such as a thin fibrous cap, large fatty or necrotic core and intraplaque hemorrhage are accepted markers of neuroischemic events. Given the systemic nature of atherosclerosis, emerging science suggests that the presence of carotid plaque is also an indicator of coronary artery plaque burden, although the preliminary data primarily involves patients with stable coronary disease. While the availability and cost-effectiveness of MRI will ultimately be important determinants of whether carotid MRI is adopted clinically in cardiovascular risk assessment, the high accuracy and reliability of this technique suggests that it has potential as an imaging biomarker of future risk.