BREMSSTRAHLUNG INTERACTIONS

Bremsstrahlung in German means ‘to brake radiation’ or braking radiation. Bremsstrahlung target interaction occurs when projectile electrons pass by outer shell electrons of target atoms and interact with the force field of the nucleus of the atom. Because atomic nuclei are positively charged and electrons are negatively charged, there is a mutual attraction between them. The nuclear force field causes the entering electron to slow down (or brake) and change direction. The loss of kinetic energy that occurs when a projectile electron slows down is emitted as an X-ray photon. These X-ray photons are known as bremsstrahlung photons or brems radiation (Fig. 9.1). In the diagnostic range, approximately 85% of X-ray emissions are the result of bremsstrahlung interactions.

Figure 9.1 Production of bremsstrahlung radiation.

CHARACTERISTIC INTERACTIONS

Characteristic target interaction occurs when projectile electrons interact with inner shell electrons of target atoms. Recall that orbital electrons within an atom have a specific binding energy. The binding energy, based on the size of the atom and the shell in which the electron is located, is the energy that would be required to remove the electron from the atom.

Characteristic radiation is produced when projectile electrons with sufficient kinetic energy eject an inner orbital electron (Fig. 9.2). When this happens, the atom becomes unstable and temporarily ionised because of the missing electron. An electron from an outer shell instantly fills the void created by the missing electron and an X-ray photon is emitted. This process continues until the atom is stable. The energy of the emitted X-ray photon is equal to the difference between the binding energy of the two involved orbital electrons. Accordingly, each X-ray photon has a specific energy level. This explains why this type of emission is called characteristic radiation. The energy emitted is characteristic of the target element and the involved shells.

Figure 9.2 Production of characteristic radiation.

Higher energy X-ray photons result with target materials of a higher proton number and interactions that involve the ejection of inner shell electrons. Each target element emits characteristic radiation of a given energy. For example, the K shell binding energy for tungsten is 69.5 keV. Only projectile electrons with energies greater than the K shell binding energy are able to eject K shell electrons. Accordingly, K shell characteristic X-rays are only produced when the applied voltage exceeds 69.5 kV_p. In comparison, the characteristic radiation of a molybdenum target (often used for mammography) is very different. The K shell binding energy for molybdenum is 20 keV,so K shell characteristic X-rays are produced when the applied voltage exceeds 20 kV_p.

With a tungsten target, only K shell interactions result in X-rays of sufficient energy to be beneficial in diagnostic radiography. All other characteristic radiation has very low energy and falls outside the useful diagnostic range. Approximately 15% of X-ray emissions are the result of characteristic interactions.

CONTINUOUS X-RAY SPECTRUM

Bremsstrahlung radiation is graphically illustrated as a continuous spectrum. The energy of a bremsstrahlung photon is the difference between the entering and exiting kinetic energy of the projectile electron. As a result, there is a continuous range of X-ray energies from zero to the maximum established by the potential difference across the X-ray tube. Maximum energy is realised if all the kinetic energy of an electron is converted into a singleX-ray photon. The maximum photon energy, determined by the maximum voltage, is the kilovolt peak (kV_p). For example, if the potential difference across the X-ray tube were 90 kV_p, an electron accelerated across the tube would attain a kinetic energy of 90 keV as it interacted with the target. If the electron transferred all of its energy, the energy of the X-ray photon would be 90 keV. The maximum photon energy is dependent on the potential difference across the tube (kV_p), regardless of the target material.

The size and shape of the emission spectrum reflects the quality and quantity of the X-ray beam. While the relative shape of the emission spectrum remains the same, its location along the horizontal axis can vary. Ranges located more towards the right represent X-ray beams of higher energy or quality. Graphically, the area under the curve represents the total number of X-rays emitted. A larger area represents X-ray beams with higher intensity or quantity. The greatest number of X-rays have approximately one-third to one-half of the maximum energy.¹

DISCRETE X-RAY SPECTRUM

Characteristic radiation is graphically illustrated in the form of a line spectrum. Remember that the energy of a characteristic photon depends on the differences between the electron binding energies of a particular target material. As a result, the spectrum produced by characteristic X-rays is referred to as discrete or distinct. For example, there are only 15 specific energy levels of characteristic X-rays from tungsten: five from interactions at the K shell, four from interactions at the L shell, and the remainder from interactions at lower energy outer shells. In tungsten, only characteristic X-rays produced from the five K shell interactions are of sufficient energy to be of diagnostic value. The number of photons produced at each characteristic energy level is different because the likelihood for filling a K shell void varies from shell to shell. Often, the five energy levels are represented on the emission spectrum as a single line. As illustrated in Figure 9.3, the vertical line at 69 keV represents the characteristic K X-rays of tungsten.

TARGET MATERIAL

The proton number of the target material affects both the quality and quantity of the X-ray beam. As mentioned previously, tungsten is the chief target material used in diagnostic radiography due to its high proton number (Z) of 74. A target material with a higher proton number results in increased production of bremsstrahlung radiation. Bremsstrahlung production is also more efficient because more high energy X-rays are produced relative to low energy X-rays. A target material with a higher proton number also results in the production of characteristic radiation of higher energy. In contrast, the molybdenum (Z = 42) and rhodium (Z = 45) targets used for mammography have much lower proton numbers. These targets produce the lower energy radiation necessary for this imaging application.

BEAM FILTRATION

Filtration of the X-ray beam affects both the quality and quantity. Beam filtration changes the characteristics of the beam by removing ineffective low energy X-rays. Inherent and added filtration reduces the quantity and increases the average energy of the X-ray beam. The result is reduced patient skin dose.

DISTANCE

The distance of the anode from the image receptor (source–image distance, SID) affects the quantity of X-rays photons (see p. 91). The inverse square law governs the relationship between the quantity of X-ray photons and the distance from the target to the image receptor. The quantity of X-ray photons at the image receptor is inversely proportional to the square of the distance from the source (see p. 91). For example, if the SID is reduced by one-half, the number of X-ray photons quadruples.

PRIME EXPOSURE FACTORS

The prime exposure factors include kV_p, mA, and exposure time. The kV_p affects both the quality and quantity, while mA and exposure time affect the quantity of the X-ray beam.

Kilovoltage (kV_p)

The kilovoltage peak (kV_p) set by the practitioner determines the voltage or potential difference applied across the cathode and anode during the exposure. This setting affects both the quality and quantity of the X-ray beam. As mentioned earlier, the kV_p setting controls the speed of the electrons travelling from the cathode to the anode. An increase in kV_p causes greater repulsion of electrons from the cathode and greater attraction of electrons towards the anode. This increased speed means projectile electrons possess greater potential energy.

Changes in kV_p affect the production of bremsstrahlung radiation, which influences both the quality and quantity of photons in the X-ray beam. An increase in kV_p results in higher quality X-ray photons with a higher average energy and more penetrating ability. Keep in mind that the maximum energy of an X-ray beam remains equal to the kV_p setting. With an increase in kV_p there is also an increase in the quantity of X-ray photons at all energy levels. However, the increase is relatively greater for high energy X-rays than for low energy X-rays. The emission spectrum in Figure 9.4 illustrates how the area under the curve increases and shifts to the right as kV_p is increased.

Figure 9.4 Changes in the emission spectrum due to increased kV_p.

Changes in kV_p also affect the production of characteristic radiation, which influences the quantity but not the quality of photons in the X-ray beam. Recall that no characteristic radiation is produced if the kV_p is less than the binding energy of the K shell electrons. For example, no characteristic radiation is produced when the applied voltage is less than 69.5 kV_p for a tungsten target because the binding energy of the K shell is 69.5 keV. However, the quantity of characteristic radiation increases when the kV_p exceeds the K shell binding energy. The increase is typically proportional to the difference between the kV_p and the binding energy.

DISTANCE

As stated previously, the inverse square law governs the relationship between X-ray quantity and distance. The formula for the inverse square law is effective when the intensity of the exposure in sieverts is known. A practical alternative is to manipulate mA s to compensate for changes in distance. The square law (a derivative of the inverse square law) calls for a change in mA s by the factor of SID².

PEAK KILOVOLTAGE (kV_p)

The kV_p controls the energy or quality of the X-rays produced at the anode and as such determines the penetrating ability of the X-rays produced. As kV_p increases, X-rays are more penetrating and as kV_p decreases, X-rays become less penetrating. The penetrating ability of X-rays also has some bearing on the number of X-rays exiting the patient. If the penetrating ability of X-ray photons is insufficient, the quantity of photons is irrelevant. In other words, increased mA s cannot compensate for inadequate kV_p.

kV_p also affects the quantity of radiation. As kV_p increases, the number of X-ray photons rapidly increases, and as kV_p decreases, the number of X-ray photons rapidly decreases. Unlike with mA s the relationship is not directly proportional. The amount of radiation produced is proportional to the square of the ratio of the kV_p. For example, if kV_p is doubled, the number of X-ray photons quadruples.

In practice, using kV_p to manage the quantity of X-ray photons is unrealistic. Again, because kV_p affects the penetrating ability of the X-ray beam, it is important to select the optimal kV_p for a given part. However, when it is necessary to vary kV_p the 15% rule is helpful. The 15% rule states that a 15% increase in kV_p will double radiographic density, whereas a 15% decrease in kV_p will halve radiographic density.

MILLIAMPERE-SECOND (mA s)

Milliampere-seconds controls the number of electrons passing from the cathode to the anode and as such controls the quantity of X-ray photons produced at the anode. There is a directly proportional relationship between the quantity of X-ray photons produced and the mA s. In other words, as mA s is increased, the number of X-ray photons increases by the same proportion and as mA s is decreased, the number of X-ray photons decreases proportionally. The law of reciprocity states that any combinations of mA and time that give the same mA s will produce the same number of X-ray photons.