Biomedical Engineering Reference
In-Depth Information
(a)
(b)
(c)
Figure 15.
Illustration delineation of endocardium of heart using different methodologies:
(a) manual segmentation by an expert; (b) semi-automated segmentation using only the
gradient information into the deformable model; (c) same as (b) but with the regional
information integrated along with geometric and gradient information. Reprinted with
permission from [35]. Copyright c
1994, IEEE.
regions are very weak. Thus, with relaxation in geometric shape the contour tends
to leak into other structures. Once the contour leaks, there is no force to stop this
leakage, and return the contour to the desired structure. The regional features come
to play in these regions of the image. The regional features are intended to prevent
leakage through fuzzy and weak boundaries, since they use attributes other than
simple local statistics. In a different form, these region-based approaches try to use
global information derived in some form or other. The contour tries to maximize
the homogeneous region enclosed by it. The region-based energy is thus defined
as [35]
ln
P
(
I
s
|
p
)=
I
s
(
x, y
)
dA,
(16)
A
where
I
s
(
x, y
) is the intensity at the pixel location (
x, y
) in the image, and the
integral gives the total area
A
enclosed by the curve
p
.
The result of using regional informationwithin a deformablemodel framework
as described in Eqs. (12) and (13) is shown in Figure 15 for segmentation of the
endocardium. As is evident, the region-based information visually improved the
segmentation quality compared to the one using only gradient information.
The homogeneity feature has also been used for segmentation of an ultrasound
image (see [32]). The external energy is defined using the homogeneity of the
region through which a control point is moving. As the curve moves from one
position to another while deforming, the position of the point also changes, and thus
the associated intensity value (provided it is not moving through a homogeneous
surface). Both the edge- and region-based energies have their own advantages and
disadvantages. Edge-based energy can give good localization of the contour near
the boundaries. Unfortunately, it has a small realm of attraction, thus requiring
