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  • 1. Radioactivity, Radionuclide Production & Radiopharmaceuticals Half-lives and transformations Cyclotrons and generators Methods of localization
  • 2. Activity
    • The quantity of radioactive material, expressed as the number of radioactive atoms undergoing nuclear transformation per unit time, is called activity (A)
    • Traditionally expressed in units of curies (Ci), where 1 Ci = 3.70 x 10 10 disintegrations per second (dps)
    • The SI unit is the becquerel (Bq)
      • 1 mCi = 37 MBq
  • 3. Decay Constant
    • Number of atoms decaying per unit time is proportional to the number of unstable atoms
    • Constant of proportionality is the decay constant (  )
      • -dN/dt =  N
      • A =  N
  • 4. Physical Half-Life
    • Useful parameter related to the decay constant; defined as the time required for the number of radioactive atoms in a sample to decrease by one half
      •  = ln 2/T p1/2 = 0.693/T p1/2
    • Physical half-life and decay constant are inversely related and unique for each radionuclide
  • 5. Fundamental Decay Equation
    • N t = N 0 e -  t or A t = A 0 e -  t
    • where:
      • N t = number of radioactive atoms at time t
      • A t = activity at time t
      • N 0 = initial number of radioactive atoms
      • A 0 = initial activity
      • e = base of natural logarithm = 2.71828…
      • = decay constant = ln 2/T p1/2 = 0.693/T p1/2
      • t = time
  • 6.  
  • 7.  
  • 8. Nuclear Transformation
    • When the atomic nucleus undergoes spontaneous transformation, called radioactive decay , radiation is emitted
      • If the daughter nucleus is stable, this spontaneous transformation ends
      • If the daughter is unstable, the process continues until a stable nuclide is reached
    • Most radionuclides decay in one or more of the following ways: (a) alpha decay, (b) beta-minus emission, (c) beta-plus (positron) emission, (d) electron capture, or (e) isomeric transition.
  • 9. Alpha Decay
    • Alpha (  ) decay is the spontaneous emission of an alpha particle (identical to a helium nucleus) from the nucleus
    • Typically occurs with heavy nuclides (A > 150) and is often followed by gamma and characteristic x-ray emission
  • 10. Beta-Minus (Negatron) Decay
    • Beta-minus (  - ) decay characteristically occurs with radionuclides that have an excess number of neutrons compared with the number of protons (i.e., high N/Z ratio)
    • Any excess energy in the nucleus after beta decay is emitted as gamma rays, internal conversion electrons or other associated radiations
  • 11.  
  • 12. Beta-Plus Decay (Positron Emission)
    • Beta-plus (  + ) decay characteristically occurs with radionuclides that are “neutron poor” (i.e., low N/Z ratio)
    • Eventual fate of positron is to annihilate with its antiparticle (an electron), yielding two 511-keV photons emitted in opposite directions
  • 13.  
  • 14. Electron Capture Decay
    • Alternative to positron decay for neutron-deficient radionuclides
    • Nucleus captures an orbital (usually K- or L-shell) electron
    • Electron capture radionuclides used in medical imaging decay to atoms in excited states that subsequently emit detectable gamma rays
  • 15. Isomeric Transition
    • During radioactive decay, a daughter may be formed in an excited state
    • Gamma rays are emitted as the daughter nucleus transitions from the excited state to a lower-energy state
    • Some excited states may have a half-lives ranging up to more than 600 years
  • 16. Decay Schemes
    • Each radionuclide’s decay process is a unique characteristic of that radionuclide
    • Majority of pertinent information about the decay process and its associated radiation can be summarized in a line diagram called a decay scheme
    • Decay schemes identify the parent, daughter, mode of decay, intermediate excited states, energy levels, radiation emissions, and sometimes physical half-life
  • 17. Generalized Decay Scheme
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  • 22.  
  • 23. Radionuclide Production
    • All radionuclides commonly administered to patients in nuclear medicine are artificially produced
    • Most are produced by cyclotrons, nuclear reactors, or radionuclide generators
  • 24. Cyclotrons
    • Cyclotrons produce radionuclides by bombarding stable nuclei with high-energy charged particles
    • Most cyclotron-produced radionuclides are neutron poor and therefore decay by positron emission or electron capture
    • Specialized hospital-based cyclotrons have been developed to produce positron-emitting radionuclides for positron emission tomography (PET)
      • Usually located near the PET imager because of short half-lives of the radionuclides produced
  • 25.  
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  • 28. Nuclear Reactors
    • Specialized nuclear reactors used to produce clinically useful radionuclides from fission products or neutron activation of stable target material
    • Uranium-235 fission products can be chemically separated from other fission products with essentially no stable isotopes (carrier) of the radionuclide present
    • Concentration of these “carrier-free” fission-produced radionuclides is very high
  • 29.  
  • 30. Neutron Activation
    • Neutrons produced by the fission of uranium in a nuclear reactor can be used to create radionuclides by bombarding stable target material placed in the reactor
    • Process involves capture of neutrons by stable nuclei
    • Almost all radionuclides produced by neutron activation decay by beta-minus particle emission
  • 31.  
  • 32. Radionuclide Generators
    • Technetium-99m has been the most important radionuclide used in nuclear medicine
    • Short half-life (6 hours) makes it impractical to store even a weekly supply
    • Supply problem overcome by obtaining parent Mo-99, which has a longer half-life (67 hours) and continually produces Tc-99m
    • A system for holding the parent in such a way that the daughter can be easily separated for clinical use is called a radionuclide generator
  • 33.  
  • 34.  
  • 35. Transient Equilibrium
    • Between elutions, the daughter (Tc-99m) builds up as the parent (Mo-99) continues to decay
    • After approximately 23 hours the Tc-99m activity reaches a maximum, at which time the production rate and the decay rate are equal and the parent and daughter are said to be in transient equilibrium
    • Once transient equilibrium has been reached, the daughter activity decreases, with an apparent half-life equal to the half-life of the parent
    • Transient equilibrium occurs when the half-life of the parent is greater than that of the daughter by a factor of ~10
  • 36.  
  • 37.  
  • 38. Secular Equilibrium
    • If the half-life of the parent is very much longer than that of the daughter (I.e., more than about 100  longer), secular equilibrium occurs after approximately five to six half-lives of the daughter
    • In secular equilibrium, the activity of the parent and the daughter are the same if all of the parent atoms decay directly to the daughter
    • Once secular equilibrium is reached, the daughter will have an apparent half-life equal to that of the parent
  • 39.  
  • 40. Ideal Radiopharmaceuticals
    • Low radiation dose
    • High target/nontarget activity
    • Safety
    • Convenience
    • Cost-effectiveness
  • 41. Mechanisms of Localization
    • Compartmental localization and leakage
    • Cell sequestration
    • Phagocytosis
    • Passive diffusion
    • Metabolism
    • Active transport
  • 42. Localization (cont.)
    • Capillary blockade
    • Perfusion
    • Chemotaxis
    • Antibody-antigen complexation
    • Receptor binding
    • Physiochemical adsorption