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Selected calculations involving radiopharmaceuticals

Selected calculations involving radiopharmaceuticals. Dr. Osama A. A. Ahmed. Selected calculations involving radiopharmaceuticals.

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Selected calculations involving radiopharmaceuticals

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  1. Selected calculations involving radiopharmaceuticals Dr. Osama A. A. Ahmed

  2. Selected calculations involving radiopharmaceuticals • Isotopes are chemically identical but physically may differ slightly in mass. Different types of atomes (nuclides) of the same chemical element, each having different number of neutrons. • Isotopes can be classified as stable or unstable. Unstable isotopes characterized by radioactive transformations, so called radioactive. The radioactive isotopes are called radioisotopes or radionuclides. • Radioisotopes are naturally occurring or artificially produced. • In the process of radioactivity, an unstable isotope undergoes changes until a stable is reached, and in the transformation it emits energy in the form of radiation. • Individual radioisotopes differ in the rate of radioactive decay, but in each case, a definite time is required for the half of the original atoms to decay. This time is called the half-life of the radioisotopes. Dr. Osama A. A. Ahmed

  3. Selected calculations involving radiopharmaceuticals • The rate of decay is always a constant raction of the total number of undecomposed atoms present. • Where N is the number of undecomposed atoms at time t and  is the decay constant or the fraction disintegrating per unit time. • the exponential decay law Dr. Osama A. A. Ahmed

  4. Selected calculations involving radiopharmaceuticals • Units of radioactivity: • absolute units, total number of atoms disintegrating per unit time. • The basic unit is the curie (Ci), that quantity of a radioisotop in which 3.7 X 1010 (37 billion) atoms disintegrate per second. • Also, millicurie (mCi) 10-3 Ci, microcurie (mCi) 10-6 Ci, and nanocurie or millimicrocurie (nCi) 10-9 Ci • The international system (SI) unit for radioactivity is the becquerel (Bq), defined as 1 disintegration per second. • Also, Kilobecquerel (kBq) 103 Bq, megabecquerel (MBq) 106 Bq, and gigabecquerel (GBq) 109 Bq • 1 Ci = 3.7 X 1010 Bq = 3.7 X 104 MBq • 1 Bq = 2.7 X 10-11 Ci • 1 MBq = 2.7 X 10-5 Ci = 2.7 X 10-2 mCi = 0.027 mCi = 27 mCi • Example: A thallous chloride Tl 201 injection has a labeled activity of 550 microcurie (mCi). Express this activity in terms of megabecquerels. • 550 mCi = 0.55 mCi • 1 mCi 37 MBq • 0.55 mCi X MBq X = 20.35 MBq Dr. Osama A. A. Ahmed

  5. Selected calculations involving radiopharmaceuticals • Example: The disintegration constant of a radioisotope is 0.02496 day-1. Calculate the half-life of the radioisotope. • = 0.693/0.02496 day-1= 27.76 or 27.8 days • Example: the half-life of 198Au is 2.7 days. Calculate the disintegration constant. • 2.7 days = 0.693/  •  = 0.693/2.7 = 0.2567 day-1 Dr. Osama A. A. Ahmed

  6. Selected calculations involving radiopharmaceuticals • Example: the original quantity of a radioisotope is given as 500 mCi (18.5 MBq)/ml. if the quantity remaining after 16 days is 125 mCi (4.625 MBq)/ml, calculate: • A) the disintegration constant • B) the half life of the radioisotope • A) •  = (2.303/16) log 500/125 = 0.08666 day -1 • B • = 0.693/0.08666 day-1= 8.0 days Dr. Osama A. A. Ahmed

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