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Fig. 2 Example of using livewire segmentation algorithm to extract image contours. The white
crosses show where the user clicks the mouse button
Fig. 3 Definition of initialization landmarks. a Landmarks extracted from the mean model of the
PDM. b Landmarks extracted from the
fl
fluoroscopic images
four anatomical landmarks that we used here are the center of the top surface of the
vertebra body, the center of the bottom surface of the vertebra body, the geometrical
center of the vertebra body, and the center of the spinal process tip. Their positions
on the mean model of the PDM are obtained through point picking or center
calculation (the center of the vertebra body is computed as the center of four
boundary landmarks along the anterior-posterior direction as shown in Fig.
3
a,
while their positions on the
fl
fluoroscopic image are de
ned through interactive
picking (see Fig.
3
a, b for details).
Let us denote those landmarks de
ned on the mean model of the PDM, i.e., the
vertebra body center, the center of the top surface of the vertebra body, the center of
the bottom surface of the vertebra body and the center of the spinal process tip, as
v
1
Mean
, v
2
Mean
, v
3
Mean
, and v
4
Mean
, respectively; and their corresponding landmarks
interactively picked from the fluoroscopic image as v
X
ray
, v
X
ray
, v
X
ray
, and
