R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

40 Medical modelling
4.5 Region selected using appropriate threshold values.
However, the effects of varying the lower threshold can be seen in Figs. 4.6
and 4.7. Notice that in Fig. 4.6 it is clear that bone is present beyond the
boundary of the selected region. However, in Fig. 4.7 we can see that other
areas, unconnected to our region of interest, have been selected. It is,
therefore, essential that thresholds be accurately selected where accuracy
is of high importance. This becomes particularly critical when very thin or
narrow objects are of interest as small changes in threshold can result in
these areas not appearing in the selected region.
4.1.2 Region growing
In order to select single anatomical structures from all of those present
within the specifi ed thresholds, a technique called region growing is typically
used. This works by allowing the user to select a single pixel within a
region already specifi ed by thresholding. The software then automatically
selects every pixel within the specifi ed thresholds that is connected to the