Computational study on phase lag of arterial-wall motion for assessment of plaque vulnerability

The wall motion of atherosclerotic plaque was analyzed using a computational method, and the effects of tissue viscoelasticity, fibrosis thickness, and lipid-core stiffness on wall displacement waveforms were examined. The viscoelasticity of plaque tissues was modeled using a time Prony series with four Maxwell elements. Computational simulation of tissue indentation tests showed the validity of the proposed viscoelastic constitutive models. Decreasing the relative moduli of the viscoelastic model reduced their viscous characteristics while enhancing the stiffness of the wall, which corresponded with the effects of decreased smooth muscle cells content. A finite-element analysis was conducted for atherosclerotic wall models and wall displacement waveforms were computed. The phase difference between the first harmonics of pressure and displacement waves was selected to represent the time delay of the wall motion. As the relative modulus decreased, the wall displacement and phase lag decreased. A thinner wall and softer lipid core corresponded to a greater wall displacement and smaller phase lag. Because the phase lag of the arterial-wall motion was smaller for the plaque with a thinner cap, lower smooth muscle cells content, and softer lipid core (all features of plaques with high rupture risk), first harmonics of pressure and displacement waves can be used as an index to assess plaque vulnerability.