One megabecquerel is 0.027027 millicurie, so a millicurie is 37 megabecquerel. These are the working units of nuclear medicine, where a patient is given a measured activity and the equipment is calibrated in whichever unit its manufacturer chose.
About the units
A megabecquerel is a million decays per second; a millicurie is a thousandth of a curie, which is 3.7 × 10⁷ decays per second. The relationship 1 mCi = 37 MBq is worth memorising outright, because it appears on every dose calibrator, syringe label and protocol in the field. Europe standardised on becquerel decades ago while American practice retained the curie, so equipment, literature and training material carry both, and a department using American-made instruments with European protocols converts routinely.
The exact factor
The exact relationship is 37 MBq per mCi. Typical administered activities: a bone scan uses 600 to 800 MBq of technetium-99m, which is 16 to 22 mCi; a myocardial perfusion study 1,100 MBq or 30 mCi; a PET scan with fluorine-18 FDG 200 to 400 MBq or 5 to 11 mCi; and a therapeutic dose of iodine-131 for thyroid cancer 3,700 to 7,400 MBq, which is 100 to 200 mCi. The therapeutic doses are two orders of magnitude above the diagnostic ones, which is why the unit and the prefix both have to be read carefully.
Where you meet this conversion
Nuclear medicine and radiopharmacy throughout. Dose calibrators measure activity before administration, radiopharmacy records track it, and regulatory reporting uses it. Because most diagnostic isotopes have short half-lives — technetium-99m is six hours, fluorine-18 is 110 minutes — activity is quoted with a reference time, and a vial's stated activity is only correct at that moment. Comparing a European protocol in MBq to an American one in mCi is a daily task in departments that use literature from both.