[en] Chemisorption or ion exchange of ⁹⁹Tc^m-MDP on the surface of hydroxyapatite crystals can facilitate bone imaging, which can be used for the diagnosis of various bone disorders. However, extraosseous uptake in soft tissue structures on bone scans has been observed in some instances. Although the exact mechanism of its occurrence is currently unclear, most researchers have proposed that its mechanism include calcification of soft tissues(e.g., dystrophic calcification, and metastatic calcification), increased regional vascularity, technical artifacts, and radiopharmaceutical binding to macromolecules in the tissues. Identifying the specific mechanisms and detecting the occurrence of extraosseous uptake on the bone scan can facilitate the detection of disorders in other systems, thus reducing examinations and enhancing the diagnostic value of the study. The present essay reviews all the related articles and illustrates the commonly encountered extraosseous uptake and its possible mechanisms. (authors)