X-rays are produced when the moving electrons strike the anode target and their kinetic energy is converted to electromagnetic energy with very short wave lengths. These x-rays show the same duality of their nature as other electromagnetic radiation such as visible light—they behave both as waves and as particles. However, the majority of the time they behave as particles (whose energy is expressed by keV).
X-ray radiation is dangerous because it is capable of ionizing the atoms in our body tissues. That is, the energy of x-rays is great enough to knock orbital electrons out of their shells, thus causing electrically imbalanced ions. This ionization disrupts molecules in our bodies. Dividing cells such as those in the body of an unborn baby are especially vulnerable to ionizing radiation.
There are two main ways in which x-rays affect our bodies: 1) by Compton scattering effect and 2) by the photoelectric effect. Each of these will be described in detail below.
When high-energy x-ray photons strike our bodies, much of their energy is scattered in a phenomenon called the Compton effect. X-ray scattering is not desirable because a) it causes fogging of the x-ray film and b) it presents a health hazard to medical staff who are in the same room with the patient during fluoroscopic examinations. The Compton effect occurs in the following way: an x-ray photon strikes an outer orbital electron in one of the atoms of the body. Some of its energy ejects this electron from its shell and thus ionizes the atom. However, the x-ray photon retains most of its former energy and may either go on to ionize other atoms or, if its energy is high enough, it may exit from the patient’s body. That is why the Compton effect causes such a health threat to other people in the room. That is also why it causes film fogging. The deflected photon strikes the film at an angle that gives no useful information about the body, but does cause unnecessary exposure of the film.
The photoelectric effect is extremely important in the exposure of x-ray film because it provides the needed contrast on the film between bone and soft tissues. It occurs most frequently when lower energy x-ray photons interact with heavy atoms. The process is very similar to that of characteristic radiation in the x-ray tube.
An x-ray photon traveling through the body strikes an inner orbit electron. The x-ray transfers all its energy to this electron, which is then ejected from its shell as a photoelectron. As in characteristic radiation, another electron in a higher energy level shell drops down to take its place and in doing so, gives off an x-ray photon with an energy equivalent to the difference of the energies between the two shells. Thus, two types of energy are given off in the photoelectric effect: the photoelectron and an x-ray photon. Both these types have rather low energies (they are called soft radiation) and are completely absorbed by the tissues they interact with. However, they are capable of further ionization before they are absorbed and therefore are dangerous to the patient.
As previously stated, the photoelectric effect occurs much more readily in atoms with high atomic numbers than in lighter ones. Since the radiation from the photoelectric effect is absorbed by the tissue it affects, that means that more x-ray energy is absorbed by bone than by soft tissues. (Bone is made of calcium with a higher atomic number than the atoms in soft tissues). In other words, if a patient was having his leg x-rayed, his femur would absorb so much of the radiation that very little would get through to the film below and it would remain clear when developed. On the other hand, the soft tissues would permit most of the x-rays to pass through to the film below, thus darkening it in these areas (Figure 4).
In order to see internal organs by means of radiography, contrast agents with high atomic numbers such as barium and iodine compounds are used. Because of the photoelectric effect, these heavy compounds
(ZBa= 56, ZI-53) absorb most of the radiation, producing ideal contrast on the film.