Absorption

As the energy of the primary x-ray beam is deposited within the atoms comprising the tissue, some x-ray photons are completely absorbed. Complete absorption of the incoming x-ray photon occurs when it has enough energy to remove (eject) an inner-shell electron. The ejected electron is called a photoelectron and quickly loses energy by interacting with nearby tissues. The ability to remove (eject) electrons, known as ionization, is one of the characteristics of x-rays. In the diagnostic range, this x-ray interaction with matter is known as the photoelectric effect.

With the photoelectric effect, the ionized atom has a vacancy, or electron hole, in its inner shell. An electron from an outer shell drops down to fill the vacancy. Because of the difference in binding energies between the two electron shells, a secondary x-ray photon is emitted (Figure 3-2). This secondary x-ray photon typically has very low energy and is unlikely to exit the patient.

The probability of total photon absorption during the photoelectric effect depends on the energy of the incoming x-ray photon and the atomic number of the anatomic tissue. The energy of the incoming x-ray photon must be at least equal to the binding energy of the inner-shell electron. After absorption of some of the x-ray photons, the overall energy or quantity of the primary beam decreases as it passes through the anatomic part.

Scattering

Some incoming photons are not absorbed but instead lose energy during interactions with the atoms comprising the tissue. This process is called scattering. It results from the diagnostic x-ray interaction with matter known as the Compton effect. The loss of energy of the incoming photon occurs when it ejects an outer-shell electron from a tissue atom. The ejected electron is called a Compton electron or secondary electron. The remaining lower-energy x-ray photon changes direction and may leave the anatomic part to interact with the image receptor (Figure 3-3).

Compton interactions can occur within all diagnostic x-ray energies and are an important interaction in radiography. The probability of a Compton interaction occurring depends on the energy of the incoming photon. It does not depend on the atomic number of the anatomic tissue. For example, a Compton interaction is just as likely to occur in soft tissue as in tissue composed of bone.

When a higher kVp is used, the overall number of x-ray interactions with matter decrease because of increased photon transmission. However, the percentage of photoelectric interactions generally decreases at higher kilovoltages within the diagnostic range, whereas the percentage of Compton interactions is likely to increase at higher kilovoltages within the diagnostic range. Box 3-1 compares photoelectric and Compton interactions.

Coherent scattering is an interaction that occurs with low-energy x-rays, typically below the diagnostic range. The incoming photon interacts with the atom, causing it to become excited. The x-ray does not lose energy, but it changes direction. Coherent scattering could occur within the diagnostic range of x-rays and may interact with the image receptor, but it is not considered an important interaction in radiography.

If a scattered photon strikes the image receptor, it does not contribute any useful information about the anatomic area of interest. If scattered photons are absorbed within the anatomic tissue, they contribute to the radiation exposure to the patient. In addition, if the scattered photon leaves the patient and does not strike the image receptor, it could contribute to the radiation exposure of anyone near the patient.

The preceding discussion focused on photon interactions that occur in radiography when using x-ray energies within the moderate range. Higher-energy x-rays, beyond the diagnostic range, result in other interactions: pair production and photodisintegration. X-ray interactions beyond the diagnostic range are important in radiation therapy.

Tissue Thickness

For a given anatomic tissue, increasing its thickness increases beam attenuation by either absorption or scattering. X-rays are attenuated exponentially and generally reduced by approximately 50% for each 4 to 5 cm (1.6 to 2 inches) of tissue thickness (Figure 3-4). More x-rays are needed to produce a radiographic image for a thicker anatomic part. Fewer x-rays are needed to produce a radiographic image for a thinner anatomic part.

Type of Tissue

Tissue composed of a higher atomic number, such as bone, attenuates the x-ray beam more than tissue composed of a lower atomic number, such as fat. The higher atomic number indicates there are more atomic particles to absorb or scatter the x-ray photon. X-ray absorption is more likely to occur in tissues composed of a higher atomic number compared with tissues composed of a lower atomic number.

Tissue density (matter per unit volume), or the compactness of the atomic particles comprising the anatomic part, also affect the amount of beam attenuation. For example, muscle and fat tissue are similar in atomic number; however, their atomic particles differ in compactness, and tissue density varies. Muscle tissue has atomic particles that are more dense or compact and therefore attenuate the x-ray beam more than fat cells. Bone is composed of tissue with a higher atomic number, and the atomic particles are more compacted or dense. Anatomic tissues are typically ranked based on their attenuation properties. Four substances account for most of the beam attenuation in the human body: bone, muscle, fat, and air. Bone attenuates the x-ray beam more than muscle, muscle attenuates the x-ray beam more than fat, and fat attenuates the x-ray beam more than air. The atomic number of the anatomic part and its tissue density affect x-ray beam attenuation.

X-ray Beam Quality

The quality of the x-ray beam or its penetrating ability affects its interaction with anatomic tissue. Higher-penetrating x-rays (shorter wavelength with higher frequency) are more likely to be transmitted through anatomic tissue without interacting with the tissues’ atomic structures. Lower-penetrating x-rays (longer wavelength with lower frequency) are more likely to interact with the atomic structures and be either absorbed or scattered. The kilovoltage selected during x-ray production determines the energy or penetrability of the x-ray photon, and this affects its attenuation in anatomic tissue. Beam attenuation is decreased with a higher-energy x-ray beam and increased with a lower-energy x-ray beam (Table 3-1).

Exit Radiation

When the attenuated x-ray beam leaves the patient, the remaining x-ray beam, referred to as exit radiation or remnant radiation, is composed of both transmitted and scattered radiation (Figure 3-6). The varying amounts of transmitted and absorbed radiation (differential absorption) create an image that structurally represents the anatomic area of interest. Scatter exit radiation (Compton interactions) that reach the image receptor do not provide any diagnostic information about the anatomic area. Scatter radiation creates unwanted exposure on the image called fog. Methods used to decrease the amount of scatter radiation reaching the image receptor are discussed in Chapter 5.

The areas within the anatomic tissue that absorb incoming x-ray photons (photoelectric effect) create the white or clear areas (more brightness or low density) on the displayed image. The incoming x-ray photons that are transmitted create areas of less brightness or high density on the displayed image. Anatomic tissues that vary in absorption and transmission create a range of brightness or shades of gray (Figure 3-7). The various shades of gray recorded in the radiographic image make anatomic tissue visible. Skeletal bones are differentiated from the air-filled lungs because of their differences in absorption and transmission.

Less than 5% of the primary x-ray beam interacting with the anatomic part actually reaches the image receptor, and an even lower percentage is used to create the radiographic image. The exit radiation that interacts with an image receptor creates the latent image, or invisible image. This latent image is not visible until it is processed to produce the manifest image, or visible image.