X-Ray Production

This chapter is derived from Carlton/Adler/Balac, Principles of Radiographic Imaging: An Art and a Science, 6th Edition (Cengage, 2020).

State the percentage of electron energy that is converted to x-ray photon energy in the x-ray tube.
Describe a bremsstrahlung target interaction.
Describe a characteristic target interaction.
Identify factors affecting characteristic K-shell photon production.
Explain the shape of the x-ray photon emission spectrum curve.

X-rays:
- Man-made processes
- Originates from the electron cloud of atoms
Gamma rays:
- Natural processes
- Originates from the atomic nucleus through radioactive decay

Increase in kinetic energy of incident electrons leads to:
- Increased quality and quantity of x-rays
- Higher number of target interactions.
Incident electrons engage in over 1,000 interactions to dissipate excess energy during x-ray production.
X-ray production is an inefficient process, with only a small conversion of kinetic energy to x-ray photons.

Target interactions occur within 0.25–0.5 mm of the target surface and include:
- Heat production
- Bremsstrahlung interactions
- Characteristic interactions

99.8% of incident electron kinetic energy is converted to heat energy.
Incident electrons transfer kinetic energy to the outer shell electrons of target atoms resulting in the emission of infrared radiation, which manifests as heat.

The term “Bremsstrahlung” means “braking rays” in German.
In these interactions, the incident electron interacts with the electrostatic force field of the atomic nucleus, leading to:
- Mutual repulsion
- Deceleration of the electron

As an incident electron decelerates due to the electrostatic forces, it emits x-ray photon energies.
This interaction accounts for the largest portion of the total x-ray beam.
Photon energy is dependent upon the proximity of the electron to the nucleus and the rate of deceleration it experiences.
- Closer proximity results in:
- Increased photon energy
- Larger deflection of the incident electron.
- Electrons that lose a small amount of kinetic energy may become part of the electrical current flow.

The largest portion of the emission spectrum is derived from Brems interactions.
Although direct interactions between the nucleus and incident electrons can yield maximum energy photons, this occurrence is rare.

In characteristic interactions, the incident electron engages with a K-shell electron.
For this interaction to be successful, the kinetic energy of the incident electron must overcome the binding energy of the K-shell electron.
- This occurs particularly in techniques using 70 kVp or higher.

If a K-shell electron is dislodged, a hole is created in the inner shell which must be filled by an electron from an outer shell.
This results in secondary photons being produced once the outer shell electron fills the gap.
Only transitions that result in electrons dropping into the K-shell will contribute to the x-ray beam as they have enough energy to be of diagnostic value.

The x-ray emission spectrum is a combination of Bremsstrahlung and characteristic emissions.
It consists of:
- A Brems hump
- A characteristic peak, which corresponds to discrete energy values based on the atomic number (Z#) of the target material.

The selected kVp controls the maximum keV possible for any emitted photon and influences spectrum attributes.
Quality versus Quantity:
- kVp controls the quality of the photons and influences the quantity:
- Quality = energy of photon
- Quantity = number of photons emitted
- Higher kVp leads to more energy in projectile electrons resulting in higher energy and more x-ray photons.
mAs (Milliampere-seconds):
- Controls quantity only:
- Quantity of x-rays = number of photons emitted.
- mAs increases by extending the time mA flows through the filament or by increasing the current flowing through the filament, leading to more photons produced but at consistent energy levels.

The average keV produced is approximately 30–40% of the selected kVp.
Characteristic peaks observed at 59 and 69 keV.
Changes in mA and kVp will alter spectrum amplitude but characteristic peak energy levels remain unchanged.

An increase in kVp modifies both beam amplitude and average energy due to increased kinetic energy imparted to incident electrons, without increasing the number of electrons striking the target.
The type of generator used can also impact the spectrum amplitude, with high-frequency generators yielding a beam with higher average energy.

The primary topics covered include the conversion of electron kinetic energy into x-ray photon energy in the x-ray tube, the two interactions resulting in the x-ray beam, the inefficiencies in heat production versus x-ray production, and the influence of various parameters, including exposure and generator types, on the x-ray photon emission spectrum curve.