Biomedical Engineering Reference
In-Depth Information
Figure 3.
Illustration of interactive editing. (a) Initial DDC showing the four user-selected
points. The portions of the DDC that do not follow the prostate boundary very well are
indicated by arrows. (b) After first deformation. (c) Three vertices (squares) are dragged
to new positions and clamped, and the DDC is deformed again. (d) After re-deformation.
Reprinted with permission from the AAPM. See attached CD for color version.
2.2.5. Parameter selection
The main parameters affecting segmentation quality are the parameters
w
img
i
in Eq. (2), and
σ
in Eq. (3a). For simplicity,
w
img
and
w
int
i
i
and
w
in
i
were assumed
to have the same value for each vertex. The weights were taken to be
w
img
i
=1
.
0
and
w
in
i
=0
.
3 for the images used in our work [31]. These valueswere empirically
selected; however, the larger value for
w
img
i
relative to
w
in
i
favors deformation of
the contour toward edges rather than smoothing due to internal forces. For noisier
images, a larger value for
w
int
i
relative to
w
img
i
would be preferable. The exact
value for the damping parameter
w
i
appears to be less critical, and
w
i
=
−
0
.
5
was found to provide good stability.
The image forces in Eq. (3b) are based on Eq. (3a). These forces act over a
limited spatial range around an edge. A vertex on the DDC can only be driven to
the closest edge in the image if the vertex is within this range. The parameter
σ
in
