Multimodality Visualization

3D visualization can be used for morphological correlation between different forms of volumetric data, such as those acquired using different types of imaging exams or modality, mostly on the same subject (Figs 5 and 6). Such spatial comparisons or localizations help to validate the efficacy of examinations. Often, direct correlation measures do not

FIGURE 5 Multiple projection 2D photographs, back projected onto the surface obtained from CT and fused with MR of the brain. (Image courtesy of M. Solaiyappan, Nick Bryan, Pheng Ann Heng.)

provide a good quantitative description of the correspondence between the data because statistical variations deteriorate the correlation measure. Multimodality volumetric visualization provides correlation information and helps the user to visualize the presence of spatial localization in the data. When the two sets of data have different spatial orientations, automated 3D registration may be difficult and interactive visualization may help explore and understand the data. The technical challenges in this class of volume rendering are not demanding, but the logic of combining two or more volumes requires attention. The data may need to be combined pixel by pixel, or pixels may need to be substituted from image to image. The latter method is particularly useful for visualization of multimodality data that typically are complementary. A combined visualization has the potential to provide more information than the individual data sets. This occurs, for instance, when one combines a CT image, which can distinguish the skull from softtissue, with an MRI image, which can resolve soft tissues but cannot show the skull (Fig. 5) [42,44,45].

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