104 Ch. 10

Chapter 10: X-Ray Production Lecture Notes (10/28/25)

X-Ray Production Overview

• X-rays are produced through interactions between projectile electrons and target atoms within an x-ray tube.

• Key Components:

  • Projectile Electrons: These are electrons originating from the cathode (C-) and are directed towards the anode (A+).

  • Target Atoms: Typically, tungsten is used as it has suitable properties for x-ray production.

Processes of X-Ray Production

1. Bremsstrahlung Radiation

• Bremsstrahlung means “braking” radiation.

• Contribution to X-Rays: Bremsstrahlung accounts for greater than 90% of diagnostic x-rays.

• Process Description:

  • An incident electron approaches a positively charged nucleus and is slowed down.

  • This interaction causes the electron to change direction due to the attractive force of the nucleus, losing kinetic energy in the process.

  • The lost kinetic energy is converted to x-ray energy.

• Case Example:

  • An incident electron has a kinetic energy (KE) of 90 keV and slows down to 10 keV after interacting with the nucleus.

  • The remaining energy of 80 keV transforms into x-ray energy.

  • The closer the electron comes to the nucleus, the greater the change in direction and the higher the energy of the resultant x-ray photon.

• Ionization Effects:

  • Bremsstrahlung radiation does not cause ionization of the tungsten atoms at the anode.

Bremsstrahlung Radiation Interactions

• The 10 keV electron may interact with other tungsten atoms, reducing its energy until it is depleted.

The X-Ray Spectrum

• The x-ray spectrum is a graphical representation of the photon energies produced during an exposure.

• **Axes:

  • X-axis:** Photon energy (keV)

  • Y-axis: Intensity (number of photons)

• Characteristics of Bremsstrahlung Spectrum:

  • Produces a continuous spectrum.

  • The maximum energy (Emax) is indicated by the kilovolt peak (kVp) set at the control panel. For example, if kVp is set at 70, Emax is 70 keV. The majority of produced photons will have energies around 23 keV.

Relationship Between kVp and keV

• kVp (Kilovolt Peak):

  • Definition: The peak voltage applied across the x-ray tube, which accelerates electrons.

• keV (Kiloelectron Volts):

  • Definition: A unit of energy measurement for x-ray photons.

  • Equation: Emax (in keV) corresponds directly to the kVp selected, implying that Emax = kVp.

Characteristic Radiation

• Process Description:

  • Occurs when an incident photon ejects an inner shell electron from the atom (either K or L shell).

  • An electron from an outer shell fills the vacancy, resulting in a characteristic cascade.

  • A characteristic x-ray photon is emitted when an outer shell electron fills the inner shell vacancy.

  • The energy of the emitted photon equals the difference in binding energies of the involved shells.

• Threshold for Characteristic Radiation:

  • The inner shell must be ejected by a photon of sufficient energy; generally, this requires a kVp setting of 70 or higher to ensure characteristic radiation occurs.

  • For instance, K-shell binding energy for tungsten is 69.53 keV.

X-Ray Beam Quality vs. Quantity

Beam Quality

• Definition: Refers to the penetrating ability of the x-ray beam, which is primarily influenced by the kVp setting.

  • A higher kVp leads to a greater average energy of photons produced.

Beam Quantity

• Definition: Refers to the intensity or the number of photons striking the patient, determined by the milliampere (mA) setting.

• Forces Controlling Quantity:

  • The mA setting determines the total number of photons produced; higher mA results in increased photon quantity.

Influences on Exposure Factors

• Factors affecting x-ray beam quantity and exposure:

  • Increased mAs: Proportionately increases image receptor exposure.

  • Increased kVp: Geometrically increases image receptor exposure.

  • Increased Distance: Geometrically reduces exposure to the receptor.

  • Increased Filtration: Reduces exposure intensity.

Factors Affecting the X-Ray Spectrum

• Target Material:

  • General Radiographic Tubes: Use tungsten, which shows discrete radiation energies of 58 and 67 keV.

  • Mammographic Tubes: Utilizes molybdenum with discrete radiation energies of 17 and 19 keV.

• kVp Considerations: Changing kVp does not affect the energy of the characteristic x-rays.

  • For example, if Emax is set at 100 keV, the resultant x-ray energy is approximately one-third, so x-ray energy is about 33 keV.

Effects on Emission Spectrum

• Various factors influence the x-ray emission spectrum:

  • mA: Increase amplitude = more photons produced.

  • kVp: Increase leads to a change in Emax. Characteristic lines appear if exposures are above 70 kVp.

  • Filtration: Effectively filters out lower energy x-rays without altering Emax.

  • Generator or Circuit Waveform:

  • Improvements generally lead to higher peak performance, improving emission outcomes.

Effect of kVp on the X-Ray Spectrum

• As kVp increases, the average energy of the emitted x-rays also rises, shifting the graph to the right and up.

Effect of mA on the X-Ray Spectrum

• Increasing mA leads to an upward shift in spectrum amplitude, reflecting the increase in x-ray output across all energy levels.

Effect of Filtration on the X-Ray Spectrum

• Addition of filtration causes the spectrum to shift to the right and downwards, indicating reduced intensity but enhanced effective energy.

Effect of Circuit Waveform on the X-Ray Spectrum

• Enhancement of generators results in a rightward and upward graph shift in the x-ray spectrum, indicating increased overall effective energy.

Summary of Factors Impacting X-Ray Emission Spectrum