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the centre lines of examined arteries. These lines are equidistant from their exter-
nal edges and one unit wide (fig. 6.). This gives us the skeleton of the given artery
which is much thinner than the artery itself, but fully reflects its topological struc-
ture. Of several skeletonising algorithms used to analyse medical images, the Pav-
lidis skeletonising algorithm [14] turned out to be one of the best. It facilitates
generating regular, continuous skeletons with a central location and one unit
width. It also leaves the fewest apparent side branches in the skeleton and the lines
generated during the analysis are only negligibly shortened at their ends. Skeleton-
ising is aimed only at making it possible to find branching points in the vasculari-
sation structures and then to introduce an unambiguous linguistic description of
individual coronary arteries and their branches. Lesions will be detected in a rep-
resentation defined in this way, even though their morphometric parameters have
to be determined based on a pattern showing the appropriate vessel, and not just
its skeleton. The centre lines of analyzed arteries produced by skeletonising them
is then searched for informative points, i.e. points where artery sections intersect
or end.
These points will constitute the vertices of a graph modelling the spatial
structure of the coronary vessels of the heart. The next step is labelling them by
giving each located informative point the appropriate label from the set of vertex
labels. In the case of terminal points, the set of vertex labels comprises abbrevi-
ated names of arteries found in coronary vascularisation. They have been defined
as in the table 1.
Table 1
The set of vertex labels
For the left coronary artery
For the right coronary artery
LCA
- left coronary artery
RCA
- right coronary artery
LAD
- anterior interventricular
branch (left anterior descending)
RM
- right marginal branch
CX
- circumflex branch
PI
- posterior interventricular branch
L
- lateral branch
RP
- right posterolateral branch
LM
- left marginal branch
If a given informative point is a branching point, then the vertex will be la-
belled with the concatenation of names of the vertex labels of arteries which begin
at this point. This way, all initial and final points of coronary vessels as well as all
points where main vessels branch into lower level vessels have been determined
and labelled as appropriate. After this operation, the coronary vascularisation tree
is divided into sections which constitute the edges of a graph modelling the exam-
ined coronary arteries. This makes it possible to formulate a description in the
form of edge labels which determine the mutual spatial relations between the pri-
mary components, i.e. between subsequent arteries shown in the analysed image.
These labels have been identified according to the following system. Mutual spa-
tial relations that may occur between elements of the vascular structure repre-
sented by a graph are described by the set of edges. The elements of this set have
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