In broad terms nuclear medicine is defined as the application of radionuclide techniques to the diagnosis and treatment of human disease. Nuclear medicine has been recognized as a medical specialty for the past decade or so.
Nuclear medicine was first used for the investigation of thyroid disease prior to the Second World War. During the past decade the field of nuclear medicine has expanded so rapidly and extensively that most practicing physicians trained prior to this growth are unaware of the numerous, valuable radioisotopic procedures now available to them.
Contemporary methods may be divided broadly into three groups. The largest division is diagnostic procedures, such as organ imaging, in which a radionuclide, in a suitable chemical form, is administered to a patient and the distribution of radioactivity in the body is determined by an external radiation detector. The second largest division of nuclear medicine utilizes radionuclide techniques to measure concentrations of hormones, antibodies, drugs, and other important substances in samples of blood or tissues. The third phase is therapy to treat disorder and restore the normal function of an organ.
In 1896, Henri Becquerel in France, discovered the use of radioactivity in Uranium. Radioactivity is defined as the property by which nuclei spontaneously decay or disintegrate by one or more discrete energy steps or transitions until a suitable stable state is reached.
Due to Becquerel’s research, Marie and pierre Curie found that both uranium and thorium possessed this property of radioactivity, also that some uranium minerals are more radioactive than uranium itself. For this work Becquerel and the Curies were jointly awarded the Nobel prize for Physics in 1903.
Marie and Pierre Curie’s work made an impact in the world of science with their discovery of one radioactive element radium, which changes into other elements. The Curie’s research gave meaning to the inner world of the atom.
An element is a basic substance consisting of atoms which are chemically alike. For many years before the discovery of radium, scientists had believed that atoms were the smallest units of matter. The word atom comes from a greek word meaning indivisible.
However, today we understand that most of an atom is empty space with particles revolving around a tiny core, or nucleus. The nucleus contains particles called protons and neutrons tightly locked together. The particles which revolve around the nucleus are called electrons.
Many elements, of which radium is one, are naturally radioactive. This means that they are made up of atoms which are unstable, that is, the nuclei, or cores of these atoms are constantly disintegrating of their own accord. In the process of disintegration or decay, the atoms automatically give off particles and radiation, and change into lighter elements.
The major forms of radiation given off by radioactive elements are alpha particles, beta particles, and gamma rays.
The alpha particle is a helium nucleus consisting of two protons and two neutrons. This particle is the same as the helium atom with the exception that there are no orbital electrons. Because there are no negative charges to neutralize the positively charged nucleus, the alpha particle possesses an electric charge of plus two upon emission. Since the particle is without electrons, it will not be satisfied until it acquires two electrons, making it an electrically neutral helium atom.
One alpha particle is a combination of two protons and two neutrons. Alpha particles shoot out from the nuclei of splitting atoms, such as those of radium, at a speed of about 10,000 miles per second. Alpha particles can be stopped by a few sheets of paper and are unable to penetrate the unbroken skin, and they rarely cause any damage.
Beta particles are very light and have a continuous energy spectrum. The maximum energy available to the beta particle from nuclear decay is called the endpoint energy. The number of beta particles emitted with this maximum energy are few. Beta particles lose most of their energy ionizing atoms along their paths. Penetrating power of beta particles is high when compared to alpha particles.
Beta particles are electrons which shoot out from certain radioactive atoms at the speed upward from 100,000 miles a second, but they can be stopped by approximately an inch of wood. Beta particles can penetrate about one-third inch of human tissue and cause severe burns.
Gamma rays are a form of radiant energy, or radiation released from the nuclei of radioactive atoms when they disintegrate. Gamma rays are part of the electromagnetic wave spectrum as radio waves, visible light waves, and x-rays. Gamma rays are very much like x-rays, except that their wavelengths are shorter. Their penetrating power is enormous. The gamma rays of radium can be stopped by thick sheets of concrete or lead. They can pass right through the human body and therefore can be extremely dangerous because they are capable of destroying cell life.
When a radium atom gives off an alpha particle, it becomes a radon atom. Radon is a gas which has a very short life. Radon is the decay product of radium.
In recent years, the words nuclide and radionuclide have fallen into disfavor. These words have been replaced by the terms isotope and radioisotope. Nuclide refers to any nucleus plus its orbital electrons. Isotopes refer to two or more forms of the same element, in that they have the same number of protons and a different number of neutrons. A radioisotope will disintegrate at a constant rate, with the time required to reach fifty percent of the original number of atoms referred to as the physical half-life. A physical half-life is a factor considered when selecting a particular isotope for certain use.
Every radionuclide has a fixed half-life ranging from seconds to years. Those used in clinical nuclear medicine have half-lives in the range of minutes to days.
Radionuclides are measured in terms of the amount of radioactive atoms that disintegrate in one second. The terms employed are referred to as curies, named after Marie Curie. One curie of radioactive material means that it would have 3.7 x 10
disintegrations per second. Chart No. 1 on the following page defines the Curie and its sub-units.