2012 Proceedings - International Tissue Elasticity Conference

058 TOWARDS THE QUANTIFICATION AND IMAGING OF STRESS USING MODEL–BASED
INTRAVASCULAR ULTRASOUND ELASTOGRAPHY.
Michael S. Richards 1 , Renato Perucchio 2 , Marvin M. Doyley 1 .
1 Electrical and Computer Engineering Department, 2 Mechanical Engineering Department,
University of Rochester, Rochester, NY, USA.
Background: The study of arterial plaque mechanics is essential to the detection and monitoring of
vulnerability. Intravascular ultrasound elastography (IVUSe) has been used for identification of plaque
components often associated with “vulnerability”, either through strain imaging or reconstruction based
elastography. However, it is the peak stresses within plaques, rather than the plaque material itself, that
is directly responsible for predicting plaque rupture [1].
Aims: This work expands upon a previously developed IVUSe reconstruction technique [2], using
intra–luminal pressure measurements and measured two component displacement vector fields to
reconstruct quantitative modulus images. The mechanical model is used to calculate the principal stress
images, which are then compared to known stresses, for simulation studies, and other vulnerable plaque
features.
Methods: We conducted simulations and phantom studies with known pressures and material
parameters to determine the accuracy of the calculated stress images and to determine the variability of
the calculation to assumed model parameters (i.e., boundary conditions, Poisson’s ratio). Simulated
vessels and vessel phantoms were designed with a soft lipid–like plaque bordering the inner lumen,
separated by a plaque cap of varying thickness. The two–component displacement vector field was
measured from the IVUS images using an image registration displacement estimator. The measured
displacements and pressures were then used to reconstruct the elastic modulus and calculate the
resulting principal stress images.
Results: Maximum principal stresses were calculated at the centroid of each finite element. Simulation
results showed good correlation for principal stresses (~25% RMS error). Phantom stress images were
normalized to an applied pressure of 1kPa for comparison. Resulting stress images show a clear stress
peak for the phantom created with a smaller cap width, which was qualitatively consistent with the
simulated results. Stress peaks are localized near the sides of the plaques in both phantoms, also
consistent with simulated results. The mean principal stress on the inner lumen for the small–capped
phantom was ~0.40kPa, and the large–capped phantom was ~0.32kPa.
Conclusions: Peak stresses were higher in simulations with smaller cap thicknesses. Phantom studies
further supported the simulation results, showing a higher peak stresses in phantoms having a thinner
cap region separating the plaque from the inner lumen under comparable loading conditions.
(a) (b) (c) (d)
Figure 1: PVA vessel phantom with soft “plaque” region. (a) shows a larger plaque with a smaller cap region and (c) a
smaller plaque with a thicker cap region. (b) and (d) Corresponding calculated maximum principal stress in
kilopascals (normalized to 1 kPa applied luminal pressure).
Acknowledgements: This work is funded by the National Heart and Lungs Research grant R01 HL088523.
References:
[1] P.D. Richardson, M.J. Davies and G.V.R. Born: Influence of Plaque Configuration and Stress Distribution on the
Fissuring of Coronary Atherosclerotic Plaques. Lancet, Vol. 334, 1989.
[2] M. S. Richards, M.M. Doyley: Investigating the Impact of Spatial Priors on the Performance of Model–Based IVUS
Elastography. Phys Med Biol, Vol. 56, No. 22, 2011.
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