Skeletal scintigraphy, commonly referred to as a bone scan (BS), is a valuable and versatile nuclear medicine tool. The examination is most commonly performed using the radiotracer Technetium-99m (Tc99m) complexed to a diphosphonate, either methylene diphosphonate (MDP) forming Tc99m-MDP or hydroxydiphosphonate (HDP) forming Tc99m-HDP. Tc99m is the most common radionuclide used in nuclear medicine for labeling because it is relatively inexpensive and has favorable characteristics for imaging such as good spatial resolution, an ideal photopeak (140 keV) for gamma cameras and possessing a relatively short half-life (6 hours) that allows adequate time for image acquisition without excessive radiation dose to patients. The Tc99m-phosphonates first entered practice in 1971 by Subramanian and colleagues with several subsequent variations produced until the development of Tc 99m-MDP in 1975, which remains the dominant radiotracer in skeletal scintigraphy. Both MDP and HDP serve as phosphate analogs which complex with the crystalline hydroxyapatite in the mineral phase of bone by a process called chemisorption. Tc99m phosphonates localize to bone in proportion to osteoblastic activity as seen at sites of bony remodeling and, to a lesser extent, localizes in proportion to blood flow and its delivery of the radiotracer. Thus, increased radiotracer uptake occurs with multiple pathologic processes such as fractures, infection, malignant disease and less commonly encountered osseous diseases like Paget disease, fibrous dysplasia, osteoid osteoma, and complex regional pain syndrome. The specificity of skeletal scintigraphy, therefore, relies heavily on an appropriate clinical history and correlation with other imaging modalities in conjunction with a thorough assessment of the radiotracer uptake pattern: mono-ostotic versus polyostotic, axial versus appendicular, peri-articular versus metaphyseal or diaphyseal, focal versus fusiform or linear.
There has been a relatively recent re-emergence of fluorine-18 sodium fluoride (F18-NaF) positron emission tomography (PET) for metabolic bone imaging, secondary to temporary shortages of Tc99m and the proliferation of PET-CT technology. A calcium analog, F18-NaF was introduced in 1963, and its 511 keV photons could be imaged with general-purpose rectilinear scanners or early positron detectors. Given the relatively short half-life of fluorine-18 (110 minutes) and its production via cyclotron, F18-NaF was not widely available. This, along with the introduction of Tc99m generators, the phosphate compounds, and gamma cameras, relegated F18-NaF to relative obscurity for the next 4 decades.
The advantages of modern F18-NaF PET-CT over the Tc99m-phosphonates BS include increased spatial resolution, improved target to background ratio, and increased sensitivity. Disadvantages include higher cost, slightly higher radiation dose, and potentially higher false positive rate due to increased uptake at sites of degenerative changes.