History
The German physicist Wilhelm Röntgen discovered the radiation known today as X-rays or Röntgen rays on 8th November 1895. He investigated the effects of radiation outside of various types of vacuum tubes using a thin aluminum “window” that allowed light to exit the tube but maintained the necessary vacuum. When he covered the window with cardboard he observed, despite the cardboard covering, fluorescence on a small screen outside the tube. While he was investigating the ability of various materials to block the rays, Röntgen saw the world’s first radiographic image, his own flickering skeleton on a special screen. In December 1895, he published his paper, “On a New Kind Of Rays.” 1
In 1896, the French physicist Henri Becquerel discovered that uranium salts emitted rays that resembled X-rays in their penetrating power. He demonstrated that this radiation did not depend on an external source of energy but seemed to be emitted spontaneously by uranium itself. The Polish physicist Marie Curie discovered other radioactive elements (polonium and radium). She postulated the theory of radioactivity2 that explains why some elements lose energy in form of radiation, transforming themselves spontaneously and “decaying” throughout the years. She also conducted the first studies on the treatment of cancer using radioactive substances.
1 Röntgen W. Ueber eine neue Art von Strahlen. Sitzungsberichte der Wuerzburger Physik.-medic. Gesellschaft, Wuerzburg, 1895.
2 Robert R. ,Curie M. New American Library, New York, 1974, p. 184.
Physics
Radiation, from the Greek “radius,” describes the phenomenon of different forms of energy that are emitted outward in all directions from a central source.
Electromagnetic waves can be imagined as photons propagating their way through space and matter. They carry a certain amount of energy, which is inversely related to the wavelength (Figure 1).
When electromagnetic waves travel through matter, the atoms within the matter absorb part of their energy. Depending on the energy and thus the wavelength of the electromagnetic radiation, the atoms may lose electrons, thereby changing their structure and becoming electrically charged (Figure 2). This phenomenon is called ionization. Only radiation with wavelengths shorter than 248 nm, which corresponds to an energy level of 5 eV (electron volts), such as UV light and X-rays, is ionizing, and can alter or damage living tissue by changing the DNA.
There are other types of ionizing radiation (Figure 3). Electrons, positrons and alpha particles, also interact strongly with electrons of atoms or molecules. Radioactive materials usually release alpha particles (nuclei of helium), beta particles, (quick-moving electrons or positrons), or gamma rays (electromagnetic radiation from the atomic nucleus). Alpha and beta particles can cause damage to organic tissue but they can be easily blocked – alpha particles by a piece of paper or the skin, and beta particles by a sheet of aluminum. It is important, though, that substances emitting alpha and beta particles do not get inside the human body.
In medical imaging different sources and types of radiation can be ionizing. The radiation used in Computed Tomography (CT) conventional radiography and angiography, for example, is electromagnetic radiation (i.e., X-rays). Positron Emission Tomography (PET), Single Photon Emission Computed Tomography (SPECT) and other nuclear imaging methods use radiation emitted during the decay of radioactive tracers (i.e., gamma rays)
Natural Radiation
Natural forms of energy such as oil, gas, etc. are the result of thousands of years of electromagnetic radiation from the sun and produced by atomic transformations at very high pressures and temperatures.
There are other sources of natural radiation, such as radon, a naturally occurring radioactive gas. Radon results from the radioactive decay of uranium. Uranium has been present since the earth was formed and has a very long half-life (4.5 billion years). Thus, radon will continue to exist indefinitely at about the same levels as it does today.1
Radon is responsible for most of the mean public exposure to ionizing radiation. In fact, it is often the single biggest contributor to the amount of background radiation an individual receives. Radon gas from natural sources can accumulate in buildings, especially in confined areas such as basements. Its concentration is variable according to location and no one can escape exposure to it. Breathing high concentrations of radon can cause lung cancer and, according to the United States Environmental Protection Agency, may even be the second most common cause of lung cancer.2
Everyone is exposed to different sources of natural radiation in daily life (Figure 4), with positive and negative aspects. The additional exposure caused by medical X-rays must be viewed within this context. Without the use of X-rays, many diseases could not be diagnosed early enough for effective treatment. When properly indicated, the use of radiation for medical imaging far outweighs the additional radiation risk.
1
1 Toxicological profile for radon. Agency for Toxic Substances and Disease Registry, U.S. Public Health Service, Atlanta, 1990.
2 A citizen’s guide to radon: the guide to protecting yourself and your family from radon. United States Environmental Protection Agency, Washington D.C., 1992.
X-ray Tube
Electromagnetic radiation used in CT has a mean energy of 50–70 keV (kilo-electron volts) and a wavelength of 0.018–0.025 nm (nanometer=10-9 m). This type of radiation is ionizing and can therefore pose a danger to organic tissue, depending on the dose.
In an X-ray tube, an electron beam striking an anode “target” produces X-rays. The beam is made up of electrons emitted from a heated cathode filament. The electrons are focused and accelerated towards the focal point by a high voltage of 40–140 kV applied between the cathode filament and the anode. The electron beam strikes the anode and part of its kinetic energy is converted into X-ray photons, while the remainder is converted into thermal radiation that heats up the anode. X-rays are emitted in all directions from the anode surface, the highest intensity being around 60° to 90° from the electron beam due to the angle of the anode. There is a small “window” that allows the X-rays to exit the tube with little attenuation while maintaining the vacuum seal required for X-ray tube operation. (Figure 5).
A generator is used to supply the X-ray tube with a controlled high voltage between the cathode and anode, as well as a controlled current to the cathode. If the current increases, more electrons will be beamed to the anode, producing more X-rays. If the voltage between cathode and anode is increased, the electrons will speed up, producing X-rays with higher energy in the anode. Hence, changing both the current (mA setting) and the high voltage (kV setting) will alter the output of the X-ray tube.
The X-ray beam is then projected onto the patient. Some of the X-rays pass through the patient while some are absorbed. In earlier times, silver bromide film was used to detect the X-rays directly. Modern radiology uses mostly digital methods to detect radiation patterns. For example, modern CT scanners employ solid-state detectors in which scintillation crystals convert the X-ray energy into visible light and semiconductor photodiodes measure the light intensity.
Radionuclides
Atomic nuclei consist of neutrons and protons. An element is defined by the number of protons its nucleus contains, while isotopes of an element vary in the number of neutrons. Nuclei are stable only when the numeric relationship between neutrons and protons is well balanced.
There are three categories of nuclear radiation, named alpha (α), beta (β) and gamma (γ). Nuclei with a surplus of neutrons frequently exhibit β decay, in which a neutron is converted to a proton, an electron (β radiation) and an antineutrino. On the other hand, nuclei with a surplus of protons frequently exhibit β+ decay, in which a proton is converted to a neutron, a positron (β+ radiation) and a neutrino. Often, additional γ radiation is emitted to lower the energy level of the nucleus. The resulting new isotope has a better-balanced number of nucleons (protons and neutrons) than the original one. Finally, alpha radiation, which consists of helium nuclei, occurs only in the radioactive decay of heavy nuclei.
For medical imaging, only isotopes with gamma or positron emission are used. Positrons have a very short range in the tissue, but upon contact with an electron, the resulting positron-electron annihilation produces two 511 keV photons (electromagnetic radiation), which can penetrate the body like gamma or X-rays. For radionuclide therapy, radiation with a short range is preferred if the isotope accumulates in the diseased tissue, in order to protect healthy tissue. This is true for isotopes emitting β radiation, α radiation or Auger electrons. The most important properties of a radioactive isotope are its half-life, type, probability and energy of the emitted radiation.