Biomedical Engineering Reference
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Fig. 5 Homogeneous ( left ) and heterogeneous ( right ) FE model predicted muscle surface ( green/
light grey ) for the supine position compared with the ground truth muscle surface ( red/dark grey )
segmented from supine MR images. The inset shows an enlarged view of the shoulder region
highlighting the mismatch in muscle deformation and the resulting deformation in the skin tissue.
The black line indicates kinematic constraints at the shoulder
6 Discussion
The results showed that heterogeneous models of the breast matched the supine
muscle and nipple locations to a greater extent than homogeneous models. This can
be attributed to the fact that homogeneous models tend to lower muscle stiffness in
the shoulder region of the breast causing more tissue to incorrectly deform towards
the axilla region. By including the muscle compartment in the models, the degree to
which the tissue was able to deform was restricted, thereby providing a better match
to the reference supine data. The relatively small improvement in mean muscle
RMSE seen in Table 1 can be attributed to the averaging of the errors across the
entire pectoral muscle surface that was visible in the MR images. However, this did
not take into account that the majority of the pectoral muscle volume lies in the
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