Research on three-dimensional (3D) x-ray computed tomography with microscopic resolution, or microtomography, which will establish techniques to measure mineral density in small medical specimens is proposed. The overall goal of this project is to demonstrate the ability to precisely reconstruct the bone density of small (2mm3) specimens with a resolution of 5 microns. Notably, a complete 3D measurement of density will be made with data recorded in a 2563 array. X-ray imaging is well suited for bone measurements since organic soft tissues are relatively transparent and mineral materials are relatively opaque. The proposed methods will be directed at examining mineral density in the composite material within individual bone trabeculae. Complex 3D patterns of different bone density exist within trabeculae and have significance in explaining phenomena associated with bone growth and bone remodeling. The proposed research will be conducted in an existing laboratory using a specialized microfocus x-ray source. Based on a theoretical description of instrument performance, we will modify the x-ray source, change the x-ray detector, and develop new data reconstruction software to achieve our performance goals. The proposed system will have capabilities not available from light microscopy or electron microscopy systems and will provide an order of magnitude improvement in performance relative to current x-ray microtomography systems. The ability of the proposed system to directly measure the three-dimensional properties of an intact specimen without sectioning is of particular value for the evaluation of bone material.