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a)
b)
c)
Fig. 5. Improvement energy component, rectangles represent 4 sample areas where
local avarage pixel intensity inside and outside the contour was considered: (a) - slice
below current segmented image, (b) - current segmented image, (c) - slice above current
segmented image
stage energy function focuses on two aspects of the contour that is sought: it
should be relatively small and should contain inside all the pixels representing
blood with injected contrast (pixels with intensity above the given threshold)
(Fig. 2a). Consequently, energy has the bigger value the larger contour is and
the more pixels above threshold stay outside the contour:
+4 x ∈C ou
C in )
n
E pre = n
(
D
( x )
(1)
(
I
)
n
(
C ou )
Here
is a strongly scaled down image after thresholding with weak distance po-
tential filter applied (Fig. 2b, Fig. 2c). The weights of the energy components are
chosen experimentally. Scaling down of the image reduces time of computations
and distance potential filter causes better contour behaviour during evolution
(there is no problem with locally constant character of objective function).
Detection of both ventricles allows to localize approximatelly interventricular
septum because it should lay inside that contour and should have significantly
lower intensity than pixels representing blood with contrast. In the presented
approach septum is represented by a line separating left and right ventricle.
This line is found using brute force search algorithm that tests each line starting
at the top edge of the image and ends at its bottom edge. This line is accepted
that has the longest part inside the prevoisuly found contour and has, at that
part, the maximum amount of points below the given threshold (Fig. 2d). In
fact, the search of this line can also be considered as an another active contour
algorithm where evolution method evaluates every possible contour to find the
optimal one.
D
3.2
Segmentation Phase
Having found the line representing interventricular septum, it is possible to find
a left and right ventricle using the similar approach described in preprocessing
phase. Here also the potential active contour can be of use. In order to find the
ventricle energy function one has to take into consideration three aspectes of the
sought contour: it should be relatively small, it shold lay on the proper side of
the septum line and it should contain inside all the pixels representing blood
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