Bone mineral density (BMD) is measured using X-ray or γ-ray, and dual energy X-ray absorptiometry (DXA) using X-ray beam is a standard method for the osteoporosis diagnosis and monitoring. Quantitative computed tomography (QCT) provides volumetric BMD as well as geometric information based on the three-dimensional CT image data, which contributes to evaluate the pathophysiology and efficacy of anti-osteoporotic agents. High resolution peripheral QCT becomes widespread, which provides volumetric BMD as well as microstructural parameters in the radius and tibia. This is the only modality to assess microstructure in the spongiosa and cortices with high resolution in vivo.