I. Introduction
Three-dimensional (3-D) data acquisition is well established for magnetic resonance imaging (MRI) and more recently for ultrasound (US). In simplest implementation, 3-D image information can be obtained from a sequential series of two-dimensional (2-D) tomographic images taken over a volume. The goal of multimodality image registration (3-D alignment) and fusion (data merging) is well established in certain areas where complementary information can be obtained. In this paper, refers to the entire process of registration and combination of data for visualization. We focus on the fusion of 3-D MRI and US information with the long term goal of gaining complementary information in a way that enhances the clinical usefulness of these modalities. Potential uses for merged or fused 3-D images include the placement of liver tumor information from MRI into US for subsequent biopsy or therapy; and the fusion of high-resolution volume MRI of brain, liver or renal lesions with US-guided tumor resection or ablation.