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Background
Bone scans have helped evaluate skeletal metabolism for more than 50 years, with the most commonly encountered radiopharmaceuticals, technetium-99m phosphonates, introduced in the 1970s, and the more recent PET agent fluorine-18 sodium fluoride, emerging in the 1990s with increased availability of PET. Bone scans are most commonly used to evaluate for osseous metastases followed by infection or hardware loosening; however, because bone scans allow visualization of the complex metabolism of the skeletal system and the physiology of mineralization, they can also evaluate various other rare disease entities and metabolic disorders. Monitoring of the mineralizing effects of systemic and musculoskeletal diseases with bone scans can allow for more nuanced treatment guidance and prognostication.

Educational Goals / Teaching Points
Provide a brief history of bone scans. Review the physiology of bone specific radiotracers focusing on Technetium-99m and Fluorine-18 agents. Appreciate the variety of indications and appearances of bone scans through interactive case-based review of rare diseases and pathology with answers and explanations appearing on mouse clicks. Explore areas of weakness of bone scans and review areas where F18 FDG PET would be the preferred imaging agent.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Technetium 99m-methyl diphosphonate bone scintigraphy, fluorine-18 Sodium Fluoride PET, fluorine-18 Fluorodeoxyglucose PET, calciphylaxis, alport’s disease – extraosseous uptake / metastatic calcification, myelofibrosis, CLL, sickle cell, thalassemia, erdheim-chester disease, gaucher disease, multiple hereditary exostoses, enchondromatosis, multiple epiphyseal dysplasia, melorheostosis, mastocytosis, pagets, and intra-operative bone scan assisted resection of osteoid osteoma.

Conclusion
Bone scans can demonstrate a wide range of metabolic and musculoskeletal pathologies beyond the common bone scan findings of osseous metastases, infection, and hardware loosening. An understanding of the subtleties of findings and the typical presentations of atypical diseases can widen the diagnostic depth and range of bone scans. Knowledge of the abilities and weakness of bone scans will enable appropriate use of nuclear medicine to ensure optimal patient outcomes.