Biomedical Engineering Reference
In-Depth Information
Fig. 26.7
Comparison of the uniaxial and the equibiaxial membrane tension response of bovine
liver capsule based on typical nominal tension-strain characteristics; the equibiaxial characteristic
is based on the analytic solution of the inflation test. Indicated by
dashed lines
is the combined fit of
the 2nd order polynomial strain energy form with
C
10
,
C
20
,
C
11
set to zero, and
C
01
=
0
.
320 N/mm
and
C
02
=
2
.
964 N/mm.
U
: uniaxial stress,
EB
: equibiaxial stress,
P-U
: response in uniaxial stress
of the 2nd order polynomial form,
P-EB
: response in equibiaxial stress of the 2nd order polynomial
form
used for liver capsule and a quasi-linear viscoelastic model for the parenchyma,
see Table
26.1
. The parameter values are consistent with typical observations from
aspiration experiments on healthy human liver tissue. The capsule was shown to
be about 3-times more significant for the local response of the organ to aspira-
tion as compared to parenchyma. Furthermore, when investigating the impact of
the capsule properties on the apex-displacement by varying the initial shear mod-
uli of capsule by
10 % of the reference apex-displacement
were observed. For corresponding changes in the parenchyma properties variations
were in the range of less than 1 %. Note however that all constitutive parameters are
well-determined for the chosen low-order constitutive models.
The effects of a tumor or diffuse cirrhosis were estimated by assigning consider-
ably stiffer constitutive behavior to particular subregions of the FE-model. Tumoral
tissue was assumed as very stiff nodules and modeled three-orders of magnitude
stiffer than the peripheral parenchymal tissue. In contrast, diffuse fibrosis was in-
cluded 10-times stiffer as the peripheral tissue, in accordance with the findings in
Sandrin et al. (
2003
). Tumors were modeled as cylindrical disks of 15 mm radius
and 5 mm height just underneath the aspirator embedded at different depths. Simi-
larly, diffuse cirrhosis was assigned to subregions of 15 mm radius and spread be-
tween varying depths. This analysis showed that aspiration measurements are only
marginally affected in case of lesions starting at a depth of more than 5 mm from
the surface.
±
25 %, changes of
±
