Micro-CT is a 3-D non-invasive high-resolution imaging tool, which is often used for visualizing anatomical features in small animals. Micro-CT generates 3-D grey-scale anatomical images based on X-ray attenuation differences between tissues. The rapid growth in micro-CT imaging systems and recent advances in micro-CT technology have made it an excellent tool for quantifying tissues, based on their CT image intensity alone. In this thesis we have imaged entire rats and quantified their whole-body composition of adipose tissue (fat), lean tissue (organs), and bone using a rapid cone-beam flat-panel micro-CT system. However, this technique is limited in situations where exogenous contrast agents are introduced, due to the similar X-ray attenuation values, which are exhibited by typical contrast agents and cortical bone. By imaging a rat specimen (containing a lead-based vascular contrast) at more than one X ray energy, we have overcome this limitation and decomposed conventional post-reconstructed micro-CT images to form 3-D independent images of vessels, bone and soft-tissue