Nuclear Medicine Technology Exam

This chapter walks through each decay mode with its equation, the change in Z and A, the nuclear medicine examples you must know, and the secondary emissions (internal conversion electrons, characteristic x-rays, Auger electrons and bremsstrahlung) that follow decay.

In alpha decay the nucleus emits an alpha particle, which is a helium-4 nucleus with two protons and two neutrons. The daughter has Z reduced by 2 and A reduced by 4.

General form: parent (A, Z) becomes daughter (A - 4, Z - 2) plus alpha particle.

In beta minus decay a neutron in the nucleus changes into a proton, and the nucleus emits an electron (the beta particle) and an antineutrino. Z increases by 1 and A stays the same.

General form: neutron becomes proton plus beta minus plus antineutrino.

The energy released is shared between the beta particle and the antineutrino, so beta particles come out with a continuous spectrum of energies from zero up to a maximum (Emax). The average beta energy is roughly one-third of Emax. This is why beta energies are quoted as "maximum" values.

In positron decay a proton changes into a neutron, and the nucleus emits a positron (a positively charged electron) and a neutrino. Z decreases by 1 and A stays the same.

Positron emission can only happen if the energy difference between parent and daughter is at least 1.022 MeV, which is the mass-energy of two electrons. Once emitted, the positron travels a short distance in tissue (its positron range, on the order of a millimeter for F-18 and several millimeters for higher-energy positrons such as those from Rb-82 or Ga-68), loses its kinetic energy, and combines with an electron. The pair annihilates, producing two 511 keV photons traveling in nearly opposite directions, about 180 degrees apart. PET scanners detect these two photons in coincidence.

In electron capture the nucleus pulls in one of its own inner-shell electrons, usually from the K shell. That electron combines with a proton to form a neutron, and a neutrino is emitted. Z decreases by 1 and A stays the same, exactly like positron decay, but no 1.022 MeV threshold is needed.

The inner-shell vacancy is filled, releasing characteristic x-rays of the daughter or Auger electrons, and the daughter nucleus often emits gamma rays as well.

Isomeric transition is the release of excess energy from a metastable nucleus without any change in Z or A. The energy usually leaves as a gamma ray. Tc-99m is the key example: it decays to Tc-99, with about 89% of decays producing a usable 140 keV gamma.

Sometimes, instead of emitting a gamma ray, the nucleus transfers its excess energy directly to an orbital electron, usually a K-shell electron, ejecting it. This is internal conversion, and the ejected electron is called a conversion electron. Its energy equals the gamma energy minus the electron's binding energy. The vacancy left behind is then filled, producing characteristic x-rays or Auger electrons. Internal conversion electrons do not reach the camera and add to patient dose, which is part of why not every Tc-99m decay yields a 140 keV photon.

Whenever an inner-shell vacancy forms (after electron capture, internal conversion or a photoelectric interaction), an outer-shell electron fills it and the energy difference leaves in one of two ways:

• A characteristic x-ray, whose energy is set by the element's shell binding energies.
• An Auger electron, where the energy is transferred to another orbital electron that is ejected instead. Auger electrons are low energy and deposit their energy very locally, which matters for dosimetry and for some emerging therapies.

Bremsstrahlung (German for "braking radiation") is different. It is produced when a fast charged particle, usually a beta particle, is slowed and deflected by the electric field of a nucleus. The lost kinetic energy leaves as an x-ray photon with a continuous range of energies. Bremsstrahlung production increases with the atomic number of the absorber and with the energy of the beta particle.

That is why pure beta emitters such as P-32, Y-90 and Sr-89 should be shielded first with low-Z material such as acrylic or plastic. If you put a high-energy beta emitter directly into a lead container, the beta particles stop in lead and generate penetrating bremsstrahlung x-rays. The standard approach is acrylic first to stop the betas, with lead outside it if needed to absorb any bremsstrahlung. Bremsstrahlung is also what allows imaging after Y-90 microsphere therapy (bremsstrahlung SPECT), even though Y-90 emits no useful gamma.

A decay scheme shows the parent at the top and daughter energy levels below. Arrows down and to the right mean Z increased (beta minus); down and to the left mean Z decreased (positron, electron capture, alpha); vertical arrows mean gamma emission with no change in Z. Branch percentages are listed.

The Mo-99 scheme is the one to picture. Mo-99 decays by beta minus. About 87.5% of those decays populate the metastable state that becomes Tc-99m, and the rest go more directly to ground-state Tc-99. Tc-99m then drops to Tc-99 by isomeric transition, emitting the 140 keV gamma.

• Alpha lowers Z by 2 and A by 4; beta minus raises Z by 1; positron and electron capture lower Z by 1; isomeric transition changes neither.
• Positron emission needs at least 1.022 MeV and ends with two 511 keV annihilation photons about 180 degrees apart.
• Electron capture and internal conversion both leave inner-shell vacancies that produce characteristic x-rays or Auger electrons.
• Bremsstrahlung rises with absorber Z, so shield pure beta emitters with acrylic or plastic first.
• About 87.5% of Mo-99 decays lead to Tc-99m, and Tc-99m decays by isomeric transition with a 140 keV gamma.