Recent advances in medical imaging have resulted in the introduction of dual-modality scanners which can simultaneously acquire two independent imaging modalities, such as positron emission tomography (PET) and computed tomography (CT) image data in a single session. These multi-dimensional PET-CT data contain both the functional and anatomical information of the human body, thus, providing the ability to identify anatomical structures of interest and then overlaying the result onto the corresponding functional structure, as an example. The utilization of the combined functional and anatomical information has been proven to be an effective approach in diagnosis and interpretation of certain medical conditions. However, the increase in image dimensions has not been accompanied with new visualization techniques, with the use of two-dimensional (2D) display being the norm. In this study, we propose a new approach to three-dimensional (3D) visualization of dual-modality PET-CT data in order to complement the 2D visualization and potentially improve medical diagnosis and interpretation. We have design and implemented a prototype visualization technique using real-time volume rendering and image fusion running on a commodity graphics card. We further propose the application of interactive segmentation of 3D anatomical structures from CT data which can be used to identify the corresponding functional structure in volume visualization.