Generally, visualizing structures from the outside is useful in understanding the 3D morphology. Sometimes it would be informative to look at the structures from the inside, such as in a vascular or bronchial tree or other tubular structures (Fig. 7). Such types of visualization can be called navigation visualization  because of their ability to provide necessary visualization for navigating through the internal structures. This class of visualization techniques became popular because of its similarity to "invitroscopic" systems such as endoscopes, bronchioscopes, or colonoscopes. In this aspect, these techniques are close to simulation-type visualization. The advantage of using navigational visualization on a 3D volume is that full spatial orientation is known at all times, unlike real endoscopes or bronchoscopes, where it may be very difficult to know the exact spatial location of the probe. Thus, it may be easier to get both the internal view and the surrounding spatial localization information, providing a "road-map" image . The volume rendering technique used in this type of visualization should address the strong perspective issues that are encountered. A detailed description of navigational visualization is presented in Chapter 45.
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