A bone scan or bone scintigraphy is a nuclear medicine imaging technique of the bone. It can help diagnose a number of bone conditions, including cancer of the bone or metastasis, location of bone inflammation and fractures (that may not be visible in traditional X-ray images), and bone infection (osteomyelitis). Nuclear medicine provides functional imaging and allows visualisation of bone metabolism or bone remodeling, which most other imaging techniques (such as X-ray computed tomography, CT) cannot. Bone scintigraphy competes with positron emission tomography (PET) for imaging of abnormal metabolism in bones, but is considerably less expensive. Bone scintigraphy has higher sensitivity but lower specificity than CT or MRI for diagnosis of scaphoid fractures following negative plain radiography. History Some of the earliest investigations into skeletal metabolism were carried out by George de Hevesy in the 1930s, using phosphorus-32 and by Charles Pecher in the 1940s. In the 1950s and 1960s calcium-45 was investigated, but as a beta emitter proved difficult to image. Imaging of positron and gamma emitters such as fluorine-18 and isotopes of strontium with rectilinear scanners was more useful. Use of technetium-99m (99mTc) labelled phosphates, diphosphonates or similar agents, as in the modern technique, was first proposed in 1971. Principle The most common radiopharmaceutical for bone scintigraphy is 99mTc with methylene diphosphonate (MDP). Other bone radiopharmaceuticals include 99mTc with HDP, HMDP and DPD. MDP adsorbs onto the crystalline hydroxyapatite mineral of bone. Mineralisation occurs at osteoblasts, representing sites of bone growth, where MDP (and other diphosphates) "bind to the hydroxyapatite crystals in proportion to local blood flow and osteoblastic activity and are therefore markers of bone turnover and bone perfusion". The more active the bone turnover, the more radioactive material will be seen. Some tumors, fractures and infections show up as areas of increased uptake. Note that the technique depends on the osteoblastic activity during remodelling and repair processes following initial osteolytic activity. This leads to a limitation of the applicability of this imaging technique with diseases not featuring this osteoblastic (reactive) activity, for example with multiple myeloma. Scintigraphic images remain falsely negative for a long period of time and therefore have only limited diagnostic value. In these cases CT or MRI scans are preferred for diagnosis and staging. Technique In a typical bone scan technique, the patient is injected (usually into a vein in the arm or hand, occasionally the foot) with up to 740 MBq of