Biomedical Engineering Reference
In-Depth Information
Fig. 7.4
Selective color mask applied on slices
In this case the differences in bone density were not taken into account. The
threshold interval was set between −824 and 2,050 Hounsfield units (HU). The neg-
ative limit of the threshold corresponds to the dark gray tone, while the positive
value corresponds to the light gray.
The threshold limits were set very close to the interval limits (−1,024 to 2,050)
due to the large variation of the gray intensity within the image.
The second mask only covers the spongy areas of the bone (see Fig.
7.4
). It was
applied manually and selectively, only over the dark gray areas, which represent the
low-density bone. The automatic threshold interval was set between −824 and 125.
The positive limit is so low because it corresponds to the cortical bone which must
not be part of the reconstruction. Using only the automatic mask generation was not
satisfactory in this case because the threshold interval includes also the values of
transition tones between vertebra's contours and background. The quality of the
resulted model is directly influenced by the mask application together with the
threshold interval.
Having the two color masks, one for the inner area of the bone and a global one,
two reconstructions were performed. The transformation of a 2D set of masks into
a single volume was made by contour interpolation. The contour interpolation is a
2D interpolation in the plane of the images that are smoothly expanded in the third
dimension. This interpolation algorithm uses the gray value interpolation within the
slices, and a linear interpolation between the contours in
Z
direction. The interpola-
tion methods give enough accurate results for the modeling purpose.
The volumes were computed and exported as a point cloud format. This format
contains the
x, y,
and
z
coordinates of the points that describe the two volumes in the
Cartesian space (see Fig.
7.5
).
For many medical applications, an exact segmentation of the vertebral column
including an identification of each vertebra is essential. However, although bone
structures show high contrast in CT images, the segmentation of individual verte-
brae is challenging [
32
] .
7.2.3
Surface Generation
The beginning of
surface phase
deals with two point cloud files of both inner and
outer vertebral volumes. The meaning of this phase is to create a certain number of
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