The need to superimpose coregistered tomographic images of a body organ obtained using different modalities presents a changeling visualization problem in biomedical imaging. Because of the usual complexity of the information represented, the diverse characteristics of each image and the independent nature of each modality, a single optimal way of portraying a combined image may not exist. A more realistic approach may be to provide several tools that the user can easily apply to interactively determine the clearest representation in each case. Such visuals must allow information from one image to be conveyed without significantly compromising the information content of the complimentary image. A description of some of the techniques that we have employed to simultaneously view functional PET images of the brain together with correlated structural images provided by MRI or CT is presented, the algorithms that define these methods are described and their effectiveness is discussed.