X-ray interactions with matter

It is helpful for the radiographer to understand the way x-ray photons interact with matter for two important reasons (Figure 7-1). First, it allows the radiographer to minimize the physical effects of x-ray photons on the patient that result in radiation dose and biologic harm. Second, an understanding of x-ray photon–body tissue interaction allows the radiographer to better manipulate how the particular anatomic area of interest appears radiographically. Minimizing harm to the patient and producing a quality radiographic image are both integral to the role and responsibility of the radiographer. X-rays may interact with matter in five different ways, depending on their energy. This chapter discusses all five, but keep in mind that only the first three occur within the range of energy used in diagnostic radiography. The chapter concludes with a discussion of differential absorption and other terms related to how the x-ray beam interacts in general with body tissues.

Classical interactions are also commonly known as coherent scattering or Thomson scattering. In this scattering event the incident x-ray photon interacts with an orbital electron of a tissue atom and changes direction. In this particular interaction the incident x-ray photon is of a rather low energy (generally less than 10 keV). When such low-energy incident photons interact with tissue atoms, they are not likely to ionize (remove orbital electrons from their shell). Instead, the atom absorbs the energy of this x-ray photon, causing excitation of the atom, and then immediately releases the energy in a new direction (Figure 7-2). Because the energy is reemitted in a new direction, it is now a scatter photon. It is of equal energy to the incident photon but travels in a new direction. Because of its low energy, most classical scatter photons are absorbed in the body through other interactions and do not contribute significantly to the image, but do add slightly to patient dose.

Compton scattering occurs throughout the diagnostic range, but generally involves moderate-energy x-ray photons (e.g., 20-40 keV). In this interaction, an incident x-ray photon enters a tissue atom, interacts with an orbital electron (generally a middle- or outer-shell electron), and removes it from its shell. In doing so, the incident photon loses up to one third of its energy and is usually deflected in a new direction (Figure 7-3). This interaction does three things: First, it ionizes the atom, making it unstable. Ionization in the body is significant because the atom is changed and may bond differently to other atoms, potentially causing biologic damage. If one of the “middle” orbital shells is involved, a characteristic cascade (outer-shell electrons filling inner-shell vacancies and emitting x-ray photons) also results, creating characteristic photons just as in the tube target. But here they are called secondary photons. These secondary photons are x-ray photons, but of a rather low-energy variety. Such photons generally contribute only to patient dose. Second, the ejected electron, called a Compton electron, or secondary electron, leaves the atom with enough energy to go through interactions of its own in adjacent atoms. The type of interaction the Compton electron undergoes depends on the energy it has and the type of atom it interacts with. Third, the incident photon is deflected in a new direction and is now a Compton scatter photon. It too, has enough energy to go through other interactions in the tissues or exit the patient and interact with the image receptor. The problem with Compton scatter interacting with the image receptor is that it is not following its original path through the body and strikes the image receptor in the wrong area. In so doing, it contributes no useful information to the image and only results in image fog. Because most scattered photons are still directed toward the image receptor and result in image fog, it is desirable to minimize Compton scattering as much as possible.

Compton scattering is one of the most prevalent interactions between x-ray photons and the human body in general diagnostic imaging and is responsible for most of the scatter that fogs the image. The probability of Compton scattering does not depend on the atomic number of atoms involved. Compton scattering may occur in both soft tissue and bone. The probability of Compton scattering is related to the energy of the photon. As x-ray photon energy increases, the probability of that photon penetrating a given tissue without interaction increases. However, with this increase in photon energy, the likelihood of Compton interactions relative to photoelectric interactions also increases.

Compton scatter photons may travel in any direction from their point of scattering. A deflection of zero degrees means no energy is transferred. Those photons scattered at 180 degrees represent maximum deflection and energy transfer. But keep in mind that the scattered photon still retains about two thirds of its energy. This is one reason the radiographer should never stand near the patient during exposure. Some Compton scatter photons exit the patient and would expose the radiographer. This is why shielding (lead aprons, lead gloves, etc.) is necessary during fluoroscopy or any procedure in which the radiographer or other health care worker may be near the patient and x-ray tube during exposure. It is important for the radiographer to remember that Compton scattering is the major source of occupational exposure.

Photoelectric interactions occur throughout the diagnostic range (e.g., 20-120 kVp) and involve inner-shell orbital electrons of tissue atoms. For photoelectric events to occur, the incident x-ray photon energy must be equal to or greater than the orbital shell binding energy. In these events the incident x-ray photon interacts with the inner-shell electron of a tissue atom and removes it from orbit. In the process, the incident x-ray photon expends all of its energy and is totally absorbed (Figure 7-4). The resulting ejected electron is called a photoelectron. The energy transfer between the incident photon and inner-shell electron is equal to the incident photon energy minus the binding energy of the orbital electron. This energy transfer constitutes the energy of the photoelectron. In soft-tissue atoms, the energy of the photoelectron is nearly equal to that of the incident x-ray photon because the binding energy of the soft tissue atom is very low and more is left over as kinetic energy for the photoelectron. In bone, the energy of the photoelectron is less because the orbital electron binding energy of bone atoms is greater and the incident x-ray photon has to expend more energy to remove it, leaving less as kinetic energy for the photoelectron. In either case, the photoelectron has enough kinetic energy to undergo interactions of its own before filling a vacancy in another atom elsewhere.

The probability of photoelectric events is directly proportional to the third power of the atomic number of the absorber. What this cubic relationship means to the radiographer is that when he or she makes small changes in the kVp setting or there are small changes in the atomic number of the tissue (due to anatomic variations or a pathologic condition), large changes in the probability of photoelectric events will result. With tissues, the higher the atomic number of the tissue atom, the greater the number of photoelectric events. Such atoms are more complex; that is, they have more electrons and stronger binding energies and are more likely to absorb the incident x-ray photon. This is why bone shows up as lighter shades on the radiographic image. In bone, more photons are absorbed, which means fewer photons are exposing the image receptor, resulting in the lighter shades of the image.

Pair production occurs only with very high–energy photons of 1.02 MeV or greater. The interaction occurs when the incident x-ray photon has enough energy to escape interaction with the orbital electrons and interact with the nucleus of the tissue atom. In this interaction, two particles are produced: a positron (positively charged electron) and an electron (Figure 7-5). For these particles to exist, they must each have energy of 0.51 MeV (the energy equivalent of an electron). If the photon has energy greater than 1.02 MeV, it is shared between the two as kinetic energy.

The last type of interaction between x-rays and matter is called photodisintegration. Photons with extremely high energies of more than 10 MeV may strike the nucleus of the atom and make it unstable. In photodisintegration the nucleus of the atom involved regains stability by ejecting a nuclear particle such as a proton, neutron, or alpha particle (Figure 7-6). Like pair production, photodisintegration does not occur in radiography because the energy levels required far exceed the kVp range used in diagnostic x-ray production.

Differential absorption is the difference between the x-ray photons that are absorbed photoelectrically and those that penetrate the body (Figure 7-7). It is called differential because different body structures absorb x-ray photons to different extents. Anatomic structures such as bone are denser and absorb more x-ray photons than structures filled with air such as the lungs.

When radiographers speak broadly about differential absorption—how the x-ray beam interacts with the tissues of the body—they may speak of transmission versus absorption. Transmission refers to those x-ray photons that pass through the body and reach the image receptor. It is desirable for some of the x-ray photons to pass through the body area of interest or no image would result. X-ray photons reaching the image receptor create the dark (less bright) shades of the image. Absorption refers to those photons that are attenuated by the body and do not reach the image receptor. Absorption has the opposite effect on the image as penetration. Recall that these photons are absorbed photoelectrically and will not reach the image receptor. This “lack” of exposure to the image receptor results in the lighter (brighter) shades of the image. It is also desirable to have some absorption; otherwise, the image would be uniformly dark.

Macromolecules are large molecules made up of thousands of atoms. When an x-ray photon interacts with one of these atoms as previously described, the energy transfer may manifest as a change to the structure of the macromolecule. The three most common effects are main-chain scission, cross-linking, and point lesions (Figure 7-8). Main-chain scission refers to a breakage of the major structure, the framework if you will, of the macromolecule itself. Cross-linking is the result of the formation of “limbs” as a result of irradiation (although these exist naturally in some macromolecules) that “stick” to adjacent parts of the macromolecule or neighboring molecules, creating unnatural framework. Point lesions are the result of damage to a single chemical bond. Think of these as a “wound” to the macromolecule that may cause a malfunction of the macromolecule and damage to the cell overall. The most sensitive of molecules is DNA. Damage similar to that previously described may occur in DNA as a result of radiation exposure and manifest as a range of responses from minor damage that is reversible to malignant response and permanent damage. Finally, because the human body is about 80% water, irradiation of water (interactions between x-ray photons and water molecules) can create harmful free radicals that then indirectly damage molecules and cells.