RADIOGRAPHY
X-ray Interactions in Diagnostic Imaging
COMPTON INTERACTIONS
This occurs throughout the diagnostic range, but generally involves moderate-energy x-ray photons. In this interaction, an incident x-ray photons 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.
The problem with Compton scattering 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.
This interaction is one of the 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 energy photon energy increases, the probability of that photon penetrating a given tissue without interaction incerases. However, with this increase in photon energy, the likelihood of Compton interactions relative to photoelectric interactions also increases.
Some Compton scattering 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 radiographeror 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
Photoelectric interactions occur throughout the diagnostic range (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 photons interacts with the inner-shell electron of a tissue atom and removes it from the orbit. In the process, the incident x-ray photon expends all of its energy and is totally absorbed. The resulting ejected electron is called 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. The probability of photoelectric interaction depends on the energy of the incident photons and the atomic number of the tissue being irradiated. The energy must be equal to or greater than the atomic number of the tissue atom, the greater the probability of photoelectric interactions.
X-rays that undergo photoelectric interaction provide diagnostic information to the image receptor. Because they do not reach the image receptor, these x-rays are representative of anatomical structures with high x-ray absorption characteristics; such structures are radiopaque. The photoelectric absorption of x-rays produces the light areas in a radiograph, such as those corresponding to bone. Note that this absorption that constitutes photoelectric interactions contributes significantly to patient dose accrued with each diagnostic image. Although some absorption is necessary to create an x-ray image, it is the radiographer’s responsibility to select technical factors that strike a balance between image quality and patient dose.
