A bone scan images the metabolic activity of the skeleton. This has traditionally been accomplished by imaging a radionuclide whose physiology closely mimics a metabolic process within bone. Emerging molecular imaging techniques used to assess bone physiology include optical scintigraphy and magnetic resonance imaging. This article focuses on nuclear bone scintigraphy, as it is the most established technique and has robust scientific evidence to support its use in clinical medicine. [1, 2, 3]
Nuclear scintigraphy of the bone commonly utilizes the radionuclides technetium-99m (Tc-99m) or fluoride-18 (F-18). Tc-99m is usually attached to medronic acid (Tc-99m MDP) and F-18 incorporated into sodium fluoride (F-18 NaF). These molecules are injected intravenously, and a nuclear camera that contains a salt crystal captures the decay of photons from the radioisotope. This is achieved through the process of scintillation or fluorescence that occurs when the photon emitted by the radionuclide hits the salt crystal within the nuclear camera. The scintillations are then digitized and converted to images for interpretation by a nuclear medicine physician.
The most common radiotracers used in bone scanning include Tc-99m MDP and F-18 NaF. Tc-99m MDP is used for gamma camera imaging. The standard adult dose is approximately 740 MBq. Tc-99m emits 140 keV gamma rays upon decay, and these gamma rays are detected by nuclear gamma cameras to allow localizing where the Tc-99m travels within the body. For imaging bone metabolism, the radionuclide is usually attached to medronic acid (methylene diphosphonate). Caution is recommended when it is used in pregnant or nursing women. There have been rare reports of allergic reactions to Tc-99m MDP, and medical equipment should be available to treat severe reactions.  The radiation exposure from a standard adult dose of 740 MBq is approximately 1.5 mSv. 
F-18 NaF is used for bone scans using positron emission tomography (PET). The standard adult dose is approximately 185 MBq. F-18 is a radionuclide that decays by positron emission. A decay positron then annihilates with an electron to form two 512-keV photons that travel in approximately opposite directions, 180° apart. PET cameras (PET scanners) are able to detect these photons and thereby map molecular flow of the F-18 NaF. There are no known adverse reactions to F-18 NaF, [6, 7] although there is a theoretical small risk from the radiation exposure of approximately 5 mSv. Caution is recommended when it is used in pregnant or nursing women. There is a theoretical risk of an allergic reaction, but these are virtually unheard of because the minuscule amounts administered are thought to be too small to trigger an allergic reaction.
There is an increased risk of radiation effects when a person's cumulative lifetime radiation exposure from diagnostic medical imaging exceeds 100 mSv. In an effort to further understand these potential risks, the International Atomic Energy Agency has launched a global effort to have individuals track their cumulative radiation exposure from medical imaging. 
Bone scans are useful in a wide range of diseases. A common reason to obtain a bone scan is in the evaluation of pain, in which a bone scan can help determine whether the source of the pain is from bone pathology or from the soft tissues. For example, a long-distance runner may have foot pain due to a fracture or a sprain. The bone scan can help determine if a bone injury or a tendon sprain is the cause of the pain.
Bone scans can also be useful in the evaluation of systemic diseases such as cancer or nonspecific widespread bone pain.