Biomedical Engineering Reference
In-Depth Information
Fig. 26.3
Typical pressure
profile applied on human liver
and corresponding tissue
response as the displacement
history of the apex point 'P',
see Fig.
26.2
'inverse problem'. This approach uses the finite element method to simulate the ex-
perimental procedure and aims at determining constitutive model parameters of the
soft tissues involved. The commercial FE-software Abaqus was applied here. An
axisymmetric model incorporating parenchyma and capsule as separate tissues is
used. Both tissues are assumed to be isotropic, homogeneous, incompressible, and
initially unstressed. Previous studies (Nava,
2007
; Mazza et al.,
2008
; Hollenstein
et al.,
2009
), identified the influence of probe geometry, contact conditions and fric-
tion, size of the modeled tissue portion, applied boundary conditions, and preload
in terms of the initial contact force applied prior to test onset; the latter leads to
probe indentation. A Matlab script was written to generate structured meshes with
a high refinement level in the contact region and near the aspirated surface. Several
transition regions were included to coarsen the mesh towards the bottom and lateral
boundary. The capsule was represented with linear membrane elements (MAX1)
with their nodes (slave) tied and prescribed to follow the nodes of the underly-
ing quadrilateral elements (master). The aspiration probe was modeled as analytic
rigid surface fully clamped at the reference point. The contact between aspirator
and tissue was left frictionless, which is not only consistent with previous findings
(Nava,
2007
; Hollenstein et al.,
2009
), but is as well physically plausible as the
capsule during the intraoperative measurements was moisturized with saline and
thus was wet and slippery. The reduced polynomial strain energy potential form
and quasi-linear viscoelasticity were used to describe the parenchyma. The capsule
was represented hyperelastic using the Rubin-Bodner strain energy potential (Rubin
and Bodner,
2002
), with parameters determined, as described in Sect.
26.2.2
.This
model formulation was shown to suitably represent tissue response in equibiaxial
stress state.
26.2.1.2 Scalar Parameters
An alternative approach providing an immediate evaluation of the measured re-
sponse is based on scalar parameters extracted directly from the pressure and apex
