Biological Impact of Radiation

The biological hazards of radiation exposure are discussed in depth by Bushberg et al. (2002b). The effects can be divided into two categories, stochastic and deterministic. Stochastic effects are random or chance occurrences, but the probability of an effect increases with dose. It is associated with low radiation dose exposures, and Bushberg et al. indicate that radiation-induced cancers and genetic effects are examples of these stochastic effects. The deterministic effects are linked with much higher doses of radiation than would normally be received in a routine medical radiographic examination. The severity of the effect also increases with the dose. Bushberg et al. state that the formation of cataracts, erythema, and hematopoietic damage are all examples of deterministic effects.

X-ray photons are high-energy forms of electromagnetic radiation that will always transfer some form of energy to whatever material it interacts with. We are most familiar with the photon's capability of knocking electrons out of the orbitals surrounding the nucleus, resulting in ionization of the atom. It is this "ionization effect" that is responsible for not only the biological damage but also for producing the image on the film.

Photons can also create excitation of outer orbital electrons. With this effect, the electron is not knocked out of its orbital, but rather it is lifted or elevated "briefly" out of its orbital ground state. As it returns to the ground state, energy is released. This is the process that is responsible for the light emitted by the intensifying screens that expose the film in the cassette.

The final type of photon energy transfer is thermal. According to Bushberg et al., although heat accounts for the majority of the energy transfer, it would require a "supralethal dose" to produce a minimal change in temperature.

Even a limited consideration of radiation-induced injury must begin at the atomic level. Our discussion will only consider three possible photon interactions. The first is known by several terms including classical, coherent, Thomson (Bushong 2008b), and Rayleigh (Bushberg 2002c). With this interaction, the incident photon interacts with the entire atom, not a specific electron. The excited atom emits a photon equal in energy, wavelength, and frequency to the incident photon, but in a different direction. Because the path of this "new" photon is different, it is termed a scatter photon and does not contribute to the formation of the image. However, since an electron was not removed, there is no ionization. According to Bushong, this interaction occurs at very low energies, below 10 keV, and therefore is primarily encountered with living patients during mammography. Recall from the previous discussion in the basic radiography section of Chapter 2 that kVp, or peak kilovoltage, is what the operator sets on the control panel of the x-ray unit. It represents the maximum kilovoltage during the exposure, whereas keV, or kiloelectron volts, represents the average energy output during the exposure. The keV is usually approximately 60% of the kVp setting. Therefore, if an exposure is taken at 55 kVp, it will produce an average energy of about 33 keV. According to Bushberg et al. (2002d), only 12% of the interactions at 30 keV can be attributed to classical scattering. Since the optimal setting for mummified and skeletal remains is 55 kVp, classical scattering will occur during these studies.

In the second interaction, termed photoelectric effect, the photon interacts with an inner shell electron of the atom and is completely absorbed. If the photon has sufficient energy, greater than the binding energy of the inner shell electron, the electron will be knocked out of orbit, resulting in ionization of the atom. Since a majority of the low-energy photons are absorbed by dense material, such as bone, the photoelectric effect is the predominant interaction at low-kV settings, and is responsible for high-contrast images. High-contrast images are composed primarily of black and white with few shades of gray. If the photon passes through the mummified remains or skeletal material unchanged but is absorbed by the phosphors comprising the intensifying screen within the cassette, the screens will fluoresce, exposing the film. Photoelectric effect is also responsible for the formation of the image in CR and DR.

In the third interaction, termed Compton effect, the incident photon interacts with an outer orbital electron. Since these electrons are held in place by lower binding energies, it is easier for them to be removed by even low-energy photons. The incident photon only expends a portion of its energy to knock the electron out of orbit, ionizing the atom. The interaction not only results in the photon losing some of its initial energy but also causes it to change direction. Because the photon's path is altered, it becomes a scattered photon and contributes to scattered radiation. Remember, the photon has not lost all its energy and is still capable of ionizing other atoms. The latter consequence presents two problems for the radiographer. If scatter radiation reaches the image receptor, the result will be degradation of the image by a decrease in the overall contrast and a rather gray, "blah" image. Restricting the area irradiated by the use of collimation is one of the methods the radiographer will employ to reduce scatter radiation from reaching the image receptor.

Another problem to consider is that if the scatter radiation reaches the individuals participating in the x-ray examination, chemical and molecular changes can take place within the individual, possibly resulting in biological damage. The radiographers will apply the cardinal rules of radiation protection to safeguard themselves, patients, and other individuals who need to be in the area from scatter radiation exposure. The cardinal rules will be discussed later; first, we give a brief discussion of the chemical and molecular changes resulting from radiation exposure.

Although radiation interactions begin at the atomic level, they quickly lead to changes at the chemical and molecular levels. The mechanism by which the image begins to form on the film is initiated primarily by light photons from the intensifying screens lysing or breaking the chemical bond between the silver atom and the remainder of the silver halide molecule embedded in the film emulsion. Similarly, biological damage begins with chemical changes that affect important biomolecules, impairing their ability to function properly. The most important biomolecule is deoxyribonucleic acid, DNA, and when radiation directly damages the structure, it is classified as a direct effect. Bushberg et al. (2002e) discusses various mechanisms that exist within cells to repair several types of DNA damage.

Indirect effect results from an x-ray photon cleaving the water molecule. Radiolysis of water can produce not only ions but also free radicals. The latter are extremely reactive, can act as a strong oxidizing agent, and have the ability to move through the cell membrane to reach the DNA molecule. In the presence of oxygen, this process is enhanced. Since approximately 80% of a living human is composed of water, free radicals are the primary cause of biological damage.

In a healthy individual, cells exposed to very low radiation doses have nearly a 100% survival rate (Bushong et al. 2008c). However, injury at the molecular level that cannot be repaired can progress through cellular to tissue damage and eventually advance to extensive organ involvement. At low levels of radiation exposure, these are the stochastic effects, and they are generally associated with a long latent period, possibly decades.

There has been some concern recently regarding DNA damage within mummified tissue. However, two factors must be remembered. First, mummified remains are dehydrated. Without the presence of water, the DNA will not be subjected to the by-products of radiolysis of water. The other point to consider is that the DNA is probably already fragmented due to the mummification process before the remains are exposed to radiation.

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