Assessment of vulnerable plaque composition by matching the deformation of a parametric plaque model to measured plaque deformation

Intravascular ultrasound (IVUS) elastography visualizes local radial strain of arteries in so-called elastograms to detect rupture-prone plaques. However, due to the unknown arterial stress distribution these elastograms cannot be directly interpreted as a morphology and material composition image. To overcome this limitation we have developed a method that reconstructs a Young´s modulus image from an elastogram. This method is especially suited for thin-cap fibroatheromas (TCFAs), i.e., plaques with a media region containing a lipid pool covered by a cap. Reconstruction is done by a minimization algorithm that matches the strain image output, calculated with a parametric finite element model (PFEM) representation of a TCFA, to an elastogram by iteratively updating the PFEM geometry and material parameters. These geometry parameters delineate the TCFA media, lipid pool and cap regions by circles. The material parameter for each region is a Young´s modulus, E_M, E_L, and E_C, respectively. The method was successfully tested on computer-simulated TCFAs (n=2), one defined by circles, the other by tracing TCFA histology, and additionally on a physical phantom (n=1) having a stiff wall (measured E_M=16.8 kPa) with an eccentric soft region (measured E_L=4.2 kPa). Finally, it was applied on human coronary plaques in vitro (n=1) and in vivo (n=1). The corresponding simulated and measured elastograms of these plaques showed radial strain values from 0% up to 2% at a pressure differential of 20, 20, 1, 20, and 1 mmHg respectively. The used/reconstructed Young´s moduli [kPa] were for the circular plaque E_L=50/66, E_M=1500/1484, E_C=2000/2047, for the traced plaque E_L=25/1, E_M=1000/1148, E_C=1500/1491, for the phantom E_L=4.2/4 kPa, E_M=16.8/16, for the in vitro plaque E_L=n.a./29, E_M=n.a./647, E_C=n.a./1784 kPa and for the in vivo plaque E_L=n.a./2, E_M=n.a./188, E_C=n.a./188 kPa.