Physics and Radiobiology of Nuclear Medicine

Units of Radioactivity

The unit of radioactivity is a curie. It is defined as

1 curie (Ci) =3.7 × 1010 disintegrations per second (dps)
=2.22 × 1012 disintegrations per minute (dpm)
1 millicurie (mCi) =3.7 × 107 dps
=2.22 × 109 dpm
1 microcurie (mCi) =3.7 × 104 dps
=2.22 × 106 dpm
The System Internationale (SI) unit for radioactivity is the becquerel
(Bq), which is defined as 1 dps. Thus,

1 becquerel (Bq) =1 dps =2.7 × 10 −^11 Ci

1 kilobecquerel (kBq) = 103 dps =2.7 × 10 −^8 Ci

2 megabecquerel (MBq) = 106 dps =2.7 × 10 −^5 Ci

1 gigabecquerel (GBq) = 109 dps =2.7 × 10 −^2 Ci

1 terabecquerel (TBq) = 1012 dps =27 Ci

Similarly,

1 Ci =3.7 × 1010 Bq =37 GBq

1 mCi =3.7 × 107 Bq =37 MBq

1 mCi =3.7 × 104 Bq =37 kBq

Specific Activity

The presence of “cold,” or nonradioactive, atoms in a radioactive sample
always induces competition between them in their chemical reactions or
localization in a body organ, thereby compromising the concentration of
the radioactive atoms in the organs. Thus, each radionuclide or radioactive
sample is characterized by specific activity, which is defined as the radio-
activity per unit mass of a radionuclide or a radioactive sample. For
example, suppose that a 200-mg^123 I-labeled monoclonal antibody sample
contains 350-mCi (12.95-GBq)^123 I radioactivity. Its specific activity would
be 350/200 =1.75 mCi/mg or 64.75 MBq/mg. Sometimes, it is confused with
concentration, which is defined as the radioactivity per unit volume of a
sample. If a 10-ml radioactive sample contains 50 mCi (1.85 GBq), it will
have a concentration of 50/10 =5 mCi/ml or 185 MBq/ml.

26 3. Kinetics of Radioactive Decay