(number of photons) in a monoenergetic beam by 50%.0.693/HVL.5 Which of the following occur during photon interactions with matter (more than one answer may apply)?Excitation.Pair production.Ionization.Compton scattering.Bremsstrahlung.Photoelectric effect.Annihilation reaction.
6 Which of the following occur during charged particle interactions with atoms (more than one answer may apply)?Excitation.Pair production.Ionization.Compton scattering.Bremsstrahlung.Photoelectric effect.Annihilation reaction.
7 Which of the following are true about annihilation reaction?The conversion of the mass of the positron and electron into energy is an example of the interchangeability of mass and energy as described by Einstein’s famous equation E = mc2.Two oppositely opposed 450 keV photons are emitted as a result of the annihilation reaction.Both (a) and (b).
8 Which of the following are referred to as nonpenetrating radiation?Positrons.Gamma photons.X‐rays.Alpha particles.Beta particles.
9 Which term refers to the loss of photons from a beam of radiation as it passes through matter?Attenuation.Absorption.
10 Which term is used to describe the transfer of energy from radiation to surrounding matter?Attenuation.Absorption.
11 You shield a sample of 99mTc using a 1 mm‐thick sheet of lead. What fraction of the photons will be transmitted through the lead? The linear attenuation coefficient, μ, of lead for 140 keV photons is 23.1 cm–1.
Answers
1 (a) and (c) are true, (b) is false; alpha particles have a shorter range than beta particles.
2 True.
3 False: Compton scattering is the dominant interaction.
4 (a) 2, (b) 1, (c) 3.
5 (b), (c), (d), (f).
6 (a), (c), (e), (g).
7 (a).
8 (a), (d), (e).
9 (a).
10 (b).
11 0.1 (transmitted fraction = Iout / Iin = e–μx = e–2.31 = 0.1).
CHAPTER 3 Formation of Radionuclides
Many radionuclides exist in nature. An example is 14C, which decays slowly with a half‐life of 5700 years and is used to date fossils. The nuclides we use in nuclear medicine, however, are not naturally occurring but rather are made either by bombarding stable atoms or by splitting massive atoms. There are three basic types of equipment that are used to make medical nuclides: generators, cyclotrons, and nuclear reactors.
Generators
Generators are units that contain a radioactive “parent” nuclide with a relatively long half‐life that decays to a short‐lived “daughter” nuclide. The most commonly used generator is the technetium‐99m (99mTc) generator (Figure 3.1), which consists of a heavily shielded column with molybdenum‐99 (99Mo; parent) bound to the alumina of the column. The 99mTc (daughter) is “milked” (eluted) by drawing sterile saline through the column into the vacuum vial. The parent 9Mo (small gray circles) remains on the column, but the daughter 99mTc (white circles) is washed away in the saline.
A generator like the one just described is frequently called a cow, the elution of the daughter nuclide is referred to as milking, and the surrounding lead is called a pig, a term used for any crude cast‐metal container. Another generator that is used extensively for cardiac imaging is the 82Sr–82Rb generator. Its design and construction is very similar to the 99Mo–99mTc generator but due to the 75 second half‐life of 82Rb, the generator must be located on site—usually right next to the PET scanner where the generator is eluted and the solution is infused into the patient in one automated step.
Table 3.1 describes the features of three common generators.
Activity curves for generators
The formal mathematical description of time‐activity behavior for parent and daughter radionuclides is complicated because it involves the competition between the accumulation (caused by decay of the parent) and decay of the daughter. The plot of the curve describing the amount of daughter nuclide in a generator following elution has two segments. The first segment traces the period of rapid accumulation of the daughter nuclide and lasts for approximately four half‐lives of the daughter nuclide (which for 99mTc is approximately 24 hours). The second segment of the curve traces what is called the period of equilibrium, during which time the amount of daughter nuclide decreases as the parent nuclide decays.
Medical radionuclide generator systems, for practical reasons, have parent half‐lives longer than their daughters—in most cases much longer. We classify generators into two groups: those in which the parent half‐life is 10 to 100 times that of the daughter and those in which the parent half‐life is more than 100 times that of the daughter. In the first group, the activity of the daughter during equilibrium decreases perceptibly over time (when time is measured in units of daughter half‐lives). This is called transient equilibrium. On the other hand, the equilibrium segment of the curve for the second group is relatively flat. This is called secular equilibrium.
Figure 3.1 99mTechnetium generator.
Table 3.1 Characteristics of three commonly used generators
| Generator (Parent–Daughter) | Clinical uses of daughter nuclide | Half‐life of parent (T1/2p) | Half‐life of daughter (T1/2d) | T1/2p/T1/2d |
|---|---|---|---|---|
| 99Mo–99mTc (molybdenum‐99–technetium‐99m) | Used in most radiopharmaceuticals for nuclear studies | 66 h | 6 h | 11 |
| 82Sr–82Rb (strontium‐82–rubidium‐82) | Cardiac perfusion imaging (PET) | 25.5 days | 75 s | 29,000 |
| 68Ge–68Ga (germanium‐82–gallium‐82) | Neuroendocrine imaging (PET) | 271 days |
68
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