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Longitudinal Mechanical Stresses in Carotid Plaques of Symptomatic Patients

Samuel A Kock¹, Jens Nygaard², Søren Dalager³, Minako Oikawa⁴, Chun Yuan⁴, Nikolaj Eldrup⁵, Anette Klærke⁵, William Paaske⁵, Won Y. Kim.^6,1
1Aarhus University Hospital Skejby, Aarhus N, Denmark;²Interdisciplinary Nano Center, Aarhus University, Aarhus N, Denmark;³Department of Pathology, Aarhus University Hospital Skejby, Aarhus N, Denmark;⁴Vascular Imaging Lab, University of Washington, Seattle, Wash.;⁵Department. of Cardiothoracic and Vascular Surgery, Aarhus University Hospital Skejby, Aarhus N, Denmark;⁶Department of Cardiology, Aarhus University Hospital Skejby, Aarhus N, Denmark.

OBJECTIVES: Due to the traumatic nature of atherosclerotic plaque rupture, it has been hypothesized that assessment of tensile stress levels inside the fibrous cap could improve carotid risk marker. Hitherto, calculating tensile stress levels has required 3D computational simulations with substantial temporal requirements limiting the clinical usefulness.

In this study, we sought to evaluate longitudinal mechanical stresses in carotid atherosclerotic plaques using a novel longitudinal 2D approach based on magnetic resonance imaging (MRI) incorporating plaque morphology, plaque geometry and local hemodynamics.

METHODS: Nineteen patients scheduled for carotid endarterectomy underwent a preoperative carotid magnetic resonance imaging (MRI) examination using a multispectral protocol to characterize plaque morphology and geometry (lipid-rich necrotic core, fibrous cap, thrombi, vessel lumen, and vessel wall). Two-dimensional longitudinal computational models were generated semi-automatically from the MRI data, and the longitudinal carotid stresses calculated.

First principal stresses and velocity field from a patient with a 70% stenosis. Maximal stresses occurred below the flow divider (inset). Multispectral MRI revealed evidence of plaque disruption coincident with the maximal stresses at the minimal fibrous cap width location (white arrow).

RESULTS: Longitudinal mechanical stresses were shown to correlate with minimal fibrous cap thickness (r=-0.61, p=0.01) and with the severity of stenosis (r=0.71, p=0.003). Maximal stress levels were asymmetrically distributed longitudinally, with 50% occurring proximal to the area of maximal stenosis, 25% level, and 25% distal, in accordance with the observed sites of plaque rupture. Plaque rupture occurred at the level of maximal first principal stresses coincident with the thinnest fibrous cap in six of seven patients with evidence of plaque disruption (top figure).

 
Correlations. A, B, and C were statistically significant.

CONCLUSIONS: In symptomatic patients with carotid atherosclerosis, the longitudinal mechanical stresses on the fibrous cap were shown to correlate with known markers of plaque vulnerability. Evaluation of longitudinal mechanical stress levels may improve risk assessment in patients with carotid atherosclerosis.

AUTHOR DISCLOSURES: S.A. Kock, Patent Pending on the Methodology Employed; J. Nygaard, Patent Pending on the Methodology Employed; S. Dalager, None; M. Oikawa, None; C. Yuan, None; N. Eldrup, None; A. Klærke, None; W. Paaske, None; W.Y. Kim, Patent Pending on the Methodology Employed.