View Full Version : Re: CT-Scan

John C. Coleman
02-04-2003, 04:10 AM
Arno Grunendahl wrote:

>As far as I understood the method, the meshing tools build a tetraedric
>mesh on the surface and somehow "deepen" the mesh to a volumetric model.

This is not entirely accurate. Volume meshing of a closed triangular
surface mesh is one way to create a 3-D model. There are others which
may be better for your particular situation. The "best" method really
depends on a number of factors -- including the shape of the bone,
the spatial resolution of the CT scan, the preferred type and number
of elements desired, and the method intended to assign material

For example, if the topology of your geometry is generally similar
from one CT image to the next, it may be relatively easy to generate
a model by stacking up sequential contours and generating a volume
based on these contours, which can then be meshed (perhaps even with
hexahedral elements). If the topology changes more dramatically
(e.g., due to branching or the presence of interior voids), then the
problem is more difficult and may require a more complicated approach.

>My question is, how can I distinguish between e.g. cortical and
>cancellous bone in a vertebrae with this precedure?

It would be helpful to have a bit more information. Do you hope to
distinguish cortical bone from cancellous bone in a geometric sense
-- i.e., having a mesh in which elements are physically either a
member of a "cortical bone" volume or a "cancellous bone" volume? As
described by an earlier poster, this process of identifying two
tissues into separate volumes is known as segmentation. Or do you
hope to differentiate between the types of bone by assigning
site-specific materials properties (based on CT density) to the
elements of a non-segmented mesh?

During my time in graduate school at Tulane University, several
graduate students developed procedures to generate FE models from CT
scans. Our basic approach was to generate the 3-D geometry from
sequential contours, mesh it, and then assign material properties to
the elements based upon CT density. (Details available at
Sometimes we simply used a threshold value of CT density to segment
the elements into cortical and cancellous bone, which were assigned
"average" material properties for those tissues. On occasion, we used
custom Fortran code to develop a completely heterogeneous FE mesh, in
which each element was assigned a unique value for Young's modulus
which was derived from the CT density.

As evidenced by the many responses to this thread, the generation of
FE models from CT scans can be a complex task. Good luck!

John C. Coleman, Ph.D.

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