Biomedical Engineering Reference
In-Depth Information
(a)
(b)
(c)
(d)
Figure 7.5: (a) Edge map after using the canny algorithm in the image of Fig. 7.3.
( b) Erosion result over the Canny algorithm output. (c) Isolated region of inter-
est. (d) Final result after dilation.
In this section, we are interested in applying
morphological chains
(sequence
of a morphological operations) techniques to isolate specific regions in medical
images. These extracted regions will be used for initializing deformable models.
We begin with a grayscale image such as in Fig. 7.3. Firstly, an edge detection
filter is applied. The Canny edge detector was used [9], despite the fact that there
are many other possibilities [13, 35, 40]. Figure 7.5(a) gives the result of applying
the Canny methodology over the image in Fig. 7.3.
In Fig. 7.5(a) note that the two white predominant regions at the center of
the image are the two lungs, which are the regions of interest. For convenience,
this image was inverted with regard to its black-white pixels before initializing
the morphological process. In this case, when applying the erosion operation
(Eq. (7.23)) over the image in Fig. 7.5(a), we eliminate artifacts, weak edges, and
separate weak connected regions. The net effect is to attenuate or eliminate high-
frequency components. In the example of Fig. 7.5(a), we used a cross-structuring
element. The result can be seen in Fig. 7.5( b).
Now, the two bigger regions are detached from the other ones, and we can
separate and treat them individually. Figure 7.5(c) shows this result.
To restore the original size of the lung, we can apply the dilation operation
(Eq. (7.24)). The result can be seen in Fig. 7.5(d).
Finally, an algorithm for isoline extraction gives the polygonal curve pictured
in Fig. 7.5(d). This curve is an approximation of the desired boundary. It can be
used as the initial curve for a deformable model.
