Biomedical Engineering Reference
In-Depth Information
(a)
(b)
(c)
Figure 17.
Illustration of using shape-based prior in the deformable model framework:
(a) shape priors with the mean shape and with a standard deviation around one parameter;
(b,c) results of [38] on synthetic images. Reprinted with permission from [38]. Copyright
c
1992, IEEE.
the cortex, which has a nearly constant width throughout its extent. The force field
is defined such that if a small disk centered at a point on the active contour rests
entirely within the ribbon, it experiences no external force; if, on the other hand, a
portion of the disk intersects adjacent tissue, the disk experiences a force drawing
it back toward the cortex.
These approaches showed some promise in the particular cases where the
shape distributions are known a priori or the solution was tailored for the specific
application and shape [39]. However, the main limitations of incorporating this
information into the active contour model is a loss of its generality and deforma-
bility within a geometric paradigm, which is probably the most attractive feature
of an active contour model. Unifying the a-priori shape information with image
data in an active contour model has been proposed by many researchers [42-46].
A separate class of compact representations of shape and image data within the
deformable model framework inspired the active shape model [40] and the active
appearance model [41].
4.2. Feature Space
The purpose of incorporating a-priori features is principally to balance the
force equation in such a way that the contour will converge fromboth the object and
background toward the interface. To strike this balance it is required to optimally
use information about object and background so that the regional features will
drive the snake from any image location toward the object-background interface.
Therefore, the image surface needs to be defined in such a manner that when the
contour lies within object class (defined in some form), the force field acts in such
a way that the energy minimization criteria are reached if and only if the contour
expands to propagate toward the interface. On the other hand, if a contour point
