Becquerel and curie (units of radioactivity)

Activity, the rate at which a radioactive sample decays, is measured in becquerels (Bq), the SI unit, where 1 Bq equals one nuclear disintegration per second. The older, non-SI unit is the curie (Ci), originally based on the activity of one gram of radium-226 and now defined by exact convention as 1 Ci = 3.7 x 10^10 Bq (37 gigabecquerels). The becquerel officially replaced the curie in the International System of Units in 1975, but the curie (and its submultiples, millicurie and microcurie) remains in everyday clinical use in the United States, while most of the rest of the world uses becquerel and its multiples (kBq, MBq, GBq).

Because radiopharmaceutical doses span many orders of magnitude, from microcurie/kilobecquerel amounts of injected tracer mass to gigabecquerel/multi-hundred-millicurie therapeutic activities, the two unit systems are used side by side in practice: a typical PET tracer dose is quoted as roughly 185-370 MBq (5-10 mCi), while a typical cycle of lutetium-177 radioligand therapy is prescribed as 7.4 GBq (200 mCi). Converting between them uses the fixed factor 1 Ci = 37 GBq, so 1 mCi = 37 MBq and 1 MBq is about 0.027 mCi.

Activity (Bq or Ci) is not the same as absorbed dose (measured in gray) or equivalent/effective dose (measured in sievert): activity describes the source, while gray and sievert describe the energy actually deposited in tissue.