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11 3D Medical Imaging

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Fig. 11.12 A CT image registered to a PET scan showing cancer of the mouth. Three orthogonal cuts through the dataset are shown along with a 3D rendering (bottom right). The PET scan (green) shows the presence of cancer whereas the CT scan shows the surrounding bone. The anatomical location of the lesion in the mandible is much clearer from the registered image than from either CT scan or PET scan alone

11.4.4 Summary

We have described the methods of registration with respect to 3D medical images. There are three considerations of any registration algorithm: the form of the aligning transformation, the similarity measure and the optimization strategy. One of the big challenges in medical imaging is that the object of interest, the human body, is composed of rigid parts, tissues which have complex mechanical properties (visco-elastic), and fluids. The section on data acquisition showed that there is no single modality which can account for all phenomena, thus registration is an essential tool, but one of its main challenges is the non-rigidity of soft tissue, which induces non-unicity. As such, non-rigid registration remains an important research topic. Figure 11.12 shows an example registration of CT and PET. CT is able to show the bony anatomy well, whereas PET shows the metabolising function of the cancerous tumour. PET is inherently lower resolution but, by registration, we are better able to see the relationship between anatomy and disease.

11.5 Segmentation

The term segmentation refers to the process of identifying or delineating a structure of interest, typically for the purpose of some further processing. For example, in medical imaging, a segmentation of the heart might be required for quantita-