Scintigraphy is a nuclear medicine procedure in which low-level radioactive substances are injected into the patient for diagnostic purposes. These include bone scintigraphy, thyroid scintigraphy, octreotide scintigraphy , and, as a further development of the procedure, single photon emission computed tomography (SPECT). For example, Tl thallium(I) chloride, technetium compounds (Tc tracer, technetium tetrofosmin), PET tracers (with radiation exposure of 1100 MBq each with O-water, 555 MBq with N ammonia, or 1850 MBq with Rb rubidium chloride) are used in myocardial scintigraphy to diagnose blood flow conditions and function of the heart muscle (myocardium). The examination with 74 MBq Thallium Chloride causes a radiation exposure of about 16 mSv (effective dose equivalent), the examination with 740 MBq Technetium-MIBI about 7 mSv. Metastable Tc is by far the most important nuclide used as a tracer in scintigraphy because of its short half-life, the 140 keV gamma radiation it emits, and its ability to bind to many active biomolecules. Most of this radiation is excreted after the examination. The remaining Tc decays rapidly to Tc with a half-life of 6 hours. This has a long half-life of 212,000 years and, because of the relatively weak beta radiation released during its decay, contributes only a small amount of additional radiation exposure over the remaining lifetime. In the United States alone, approximately seven million individual doses of Tc are administered each year for diagnostic purposes.