R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

Working with medical scan data 43
4.8 Thresholding of CT data (see also colour section).
As described earlier, in CT images the grey scale is proportional to the
density of the tissue. Therefore, the denser the tissue is, the lighter the
shade of grey that corresponds to it will be. Mimics, and software like it,
uses these grey scale values to differentiate between different tissue types.
By selecting upper and lower grey scale values, specifi c tissue types can be
selected. These levels are typically referred to as thresholds. On importing
a new data set, Mimics displays the images using a default threshold for
bone, which is shown in green (see Fig. 4.8 in the colour section). Selecting
the desired tissue type is accomplished by varying the upper and lower
thresholds until the required tissue type is isolated. This process is usually
referred to as segmentation.
Once the desired tissue type, in this case bone, has been segmented, it
may be necessary to limit the selected data to one particular structure.
Region growing allows the user to select a certain pixel within the desired
structure and the software then automatically selects all other pixels that
are connected to it. As this function operates in all three dimensions, single
structures can quickly be segmented from the whole data set. This can be
seen in the yellow structure highlighted in Fig. 4.9 (see Fig. 4.9 in the colour
section).
The software can also be used to view three-dimensional shaded images
of the selected data. The segmented data is then exported in the format
used to create the computer fi les necessary to build the medical model as
shown in Fig. 4.10 (see Fig. 4.10 in the colour section).