2012 Proceedings - International Tissue Elasticity Conference

039 IMAGING VULNERABLE PLAQUES WITH ACOUSTIC RADIATION FORCE IMPULSE (ARFI)
IMAGING: FEM SIMULATION RESULTS.
JR Doherty 1 , GE Trahey 1 and ML Palmeri 1 .
1 Duke University, Durham, NC, USA.
Background: Plaque rupture is the most common cause of cardiovascular related events including
sudden cardiac death and stroke. Histopathological evidence has shown that a vulnerable plaque is
typically characterized by a large lipid core and a thin fibrous cap. However, current evaluation methods
are unable to visualize these features in vivo and cannot reliably identify patients most at risk. With the
ability to characterize the mechanical properties (i.e., stiffness) of structures deep within the body,
Acoustic Radiation Force Impulse (ARFI) Imaging [1] is a non–invasive ultrasound–based elasticity
imaging method that has shown promise for identifying soft, lipid pools from stiffer, more stable regions.
Aims: The purpose of this work is to simulate ARFI imaging of carotid plaques using Finite Element
Method (FEM) models. The ability of ARFI imaging to identify the lipid pool is evaluated across a wide
range of material properties. Where rupture of plaque could lead to serious consequences, the maximum
Von Mises stress associated with the induced deformation is also investigated for each simulation.
Methods: FEM meshes of five carotid plaques were constructed based on in vivo high resolution magnetic
resonance imaging (MRI) and histology images published by Li et al. [2]. In a parametric analysis, the
specified Young’s Modulus assigned to the media, lipid and fibrous cap components was modified across
a wide range of values. Following the work performed by Palmeri et al. [3], an ARFI excitation was
modeled by scaling simulated pressure fields with empirically determined intensity values. The material
was modeled as a linear, isotropic and elastic solid. LS–DYNA (Livermore Software Technology Corp,
Livermore, CA) was used to perform the FEM simulation to obtain datasets of the axial displacements and
Von Mises stresses that occur through time from the impulsive acoustic radiation force excitation.
Results: As shown in Figure 1, the ability to detect the lipid pool is largely dependent upon the stiffness
of the lipid component. An increased contrast was observed with an increase in the ratio of the lipid pool
stiffness to that of the media and fibrous cap. Stress concentrations from the induced deformation were
largely concentrated within the fibrous cap and media components. Increased fibrous cap stiffness
resulted in an increased Von Mises stress as shown in Figure 2. Maximum stresses were located at the
edges of the acoustic radiation force excitation and were