INTRODUCTION AND OVERVIEW

CONTENTS

   In the previous section we showed that certain nuclei are not completely stable and eventually undergo an internal change that will produce a more stable nuclear structure. This spontaneous change is a radioactive transition. In some older literature this event is called a nuclear disintegration. The terminology is misleading because the nucleus does not disintegrate; it simply undergoes a slight change. This event is illustrated in the figure below. The original nucleus is designated the parent, and the nucleus after the transition is designated the daughter. In radioactive transitions, energy is emitted as radiation. The types of radiation encountered in nuclear medicine are shown below. The radiation is in the form of either energetic particles or photons.

Various Radiations Produced by Radioactive Transitions

   Most radionuclides emit a combination of radiations; the types depend on the physical characteristics of the nucleus and are considered in more detail later. The daughter nucleus can be either stable or a radioactive or metastable nucleus that will undergo another transition in the future.

   In most in-vivo nuclear medicine procedures it is desirable to use a radionuclide that emits photons in the range of 100 to 500 keV. The penetrating ability of photons is related to their energy. Many photons in this energy range can emanate from the body, but not penetrate through the detector and be lost. Particle radiation is not useful in most diagnostic procedures. In fact, it is usually undesirable because it deposits its energy in the body close to the site of origin and can contribute significantly to patient dose without contributing to diagnostic information. With many radionuclides, particle radiation is a byproduct of the transition required for desirable photon emissions.

   It is often useful to construct a diagram to show changes in the nucleus and the radiation emitted during a radioactive transition. A transition diagram, sometimes referred to as a decay scheme, is shown below. Two types of changes occur within a nucleus: loss of energy and, possibly, a change in atomic number in an isobaric transition. In the transition diagram, relative nuclear energy is represented by vertical distance and relative atomic number by horizontal distance. The actual scales are not usually shown as they are in the figure below. The position of the nucleus before and after the transition is represented by horizontal lines in the diagram. In the figure below the image of a nucleus is resting on these lines. This is not generally found in the conventional diagram but is here to help us follow the transition. The steps in the transition are represented by lines running downward. The transition always moves downward because the nucleus is decreasing its energy by emitting radiation. If the atomic number changes (isobaric transition), the transition line will slant to the right or left.

Diagram Showing Radioactive Transition

   The vertical distance between the parent and daughter positions represents the total transition energy. This value is always specific for the transition associated with each radioactive nuclide. But all nuclei of the same nuclide do not necessarily go from the parent to daughter state in the same way. The significance of this is discussed in detail later.

ISOMERIC TRANSITIONS

CONTENTS

   After a radioactive nucleus undergoes an isobaric transition (beta emission, positron emission, or electron capture), it usually contains too much energy to be in its final stable or daughter state. Nuclei in these intermediate and final states are isomers, since they have the same atomic and mass numbers. Nuclei in the intermediate state will undergo an isomeric transition by emitting energy and dropping to the ground state.

   Gamma Emission

CONTENTS

   In most isomeric transitions, a nucleus will emit its excess energy in the form of a gamma photon. A gamma photon is a small unit of energy that travels with the speed of light and has no mass; its most significant characteristic is its energy. The photon energies useful for diagnostic procedures are generally in the range of 100 keV to 500 keV.

   The energy of a gamma photon is determined by the difference in energy between the intermediate and final states of the nucleus undergoing isomeric transition. This difference is the same for all nuclei of a specific nuclide. However, many nuclides have more than one intermediate state or energy level. When this is the case, a radionuclide might emit gamma photons with several different energies. This is illustrated in below.

Relationship of Nuclear Energy Levels to the Energy Spectrum of Gamma Photons

   The nuclide used in this illustration has two intermediate states or energy levels. One has an energy 500 keV above the daughter level, and the other is 300 keV higher than the first. When there are several different intermediate energy levels, it is common for some nuclei to go to one level and other nuclei to go to another level during isobaric transition. This is usually indicated on the transition diagram by showing the percentage of nuclei that go to each energy level. In the illustration considered, 80% of the nuclei go directly to intermediate energy level number 1, and 20% go directly to level number 2. The gamma photons are emitted when the nuclei move from these intermediate energy levels down to the daughter nuclide level.

   Nuclei that have gone to a specific intermediate energy level might then go directly to the daughter level or to a lower intermediate level. With this in mind we can predict the gamma photon energy spectrum produced by our example nuclide. The spectrum will consist of three discrete energies as shown above. Sixty percent of the parent nuclei will go to intermediate energy level number 1 and then directly to the daughter level 800 keV below. Therefore, 60% of the transitions will give rise to an 800-keV photon. Twenty percent of the nuclei that go to energy level 1 will then go to intermediate level number 2 by emitting a 300-keV photon. Forty percent of the nuclei will go through intermediate energy level number 2, either directly from the parent or from intermediate level number l. When these nuclei drop to the daughter energy level, a 500-keV gamma photon will be emitted. It is the combination of different energy levels and different transition routes that gives rise to the different energies in the typical gamma spectrum. For most radionuclides, one or two gamma energies will account for the vast majority of transitions.

   For most nuclides, the time spent by the nucleus in the intermediate state is extremely short and the isomeric transition appears to coincide with the isobaric transition. In some nuclides, however, the nuclei remain in the intermediate state for a longer time. In this case, the intermediate state is referred to as a metastable state. Metastable states are of particular interest in nuclear medicine because they make possible the separation of electron and photon radiation. In a diagnostic procedure it is undesirable to have electron radiation in the body because it contributes to radiation dosage but not to image formation. By using a nuclide that has already undergone an isobaric (electron-emitting) transition and is in a metastable state, it is possible to have a radioactive material that emits only gamma radiation.

   Technetium-99m is a nuclide used in the metastable state. The parent nuclide is molybdenum, which undergoes an isobaric transition to technetium-99 in the metastable state. The technetium-99m will later undergo an isomeric transition to technetium-99.

   Internal Conversion

CONTENTS

   Under some conditions, the energy from an isomeric transition can be transferred to an electron within the atom. This energy supplies the binding energy and expels the electron from the atom. This process is known as internal conversion (IC) and is an alternative to gamma emission. In many nuclides, isomeric transitions produce gamma photons and IC electrons. When an electron is removed from the atom by internal conversion, a vacancy is created. When the vacancy is filled by an electron from a higher energy level, energy must be emitted from the atom as a characteristic x-ray photon or an Auger electron.

The various isobaric and isomeric transitions give rise to a combination of both photon and particulate radiations. The radiations encountered in clinical procedures are summarized below. Nuclei with a high N/P generally produce beta radiation; those with a low N/P produce either positrons or electron capture. All transitions are usually followed by either gamma or internal conversion electron emission. Internal conversion and electron capture lead to x-ray or Auger electron emission.

ALPHA EMISSION

CONTENTS

   Some radioactive materials emit alpha particles during their transformation. An alpha particle consists of two neutrons and two protons. Because of their size and electrical charge, alpha particles are not good penetrators and deposit their energy very close to their origin. Because of this, and the fact that alpha particles generally have more energy than other radiation forms, significant tissue doses can result from an alpha emitter within the body. Alpha emitters are generally not used in clinical medicine. An exception is radium, which is often used in therapeutic procedures. In these procedures the radium is contained in sealed metal capsules that absorb alpha radiation and let gamma radiation through to the tissue.

   The transitions and principal radiations emitted by many nuclides used in clinical medicine are shown in the following table.

Transition