X-rays are produced when moving charged particles collide with a material (target) and are deviated from their original path, or a change in electron layers in the atom is made.

The image below shows an overview of an X-ray tube and its components in a vacuum glass envelope. In 1913 William David Coolidge  developed this tube and since then, this type of tube is also known as a Coolidge tube.

When the electric current travels through the filament (Cathode), which is often made from tungsten, the filament is heated, and the thermionic phenomenon causes electrons to detach from the filament. The number of ionizations is closely related to the temperature of the cathode. The electron cloud is accelerated toward the anode under the influence of a potential in the order of kilo voltages (KV). Anode is usually metallic and can be made of tungsten, molybdenum, or silver. When electrons collide with the anode, X-rays are generated. The number of these electrons is expressed in milliamperes (mA) or microamperes (µA). The kinetic energy of these electrons is proportional to the applied potential and is therefore expressed in kilo electron volts (KeV). Voltage, current, and exposure time are user modifiable.

What happens exactly when the electrons collide with the target?

Types of X-ray production

bremsstrahlung:

As electrons collide with the target, they deviate from their primary path causing their speed and therefore their kinetic energy to be reduced. This reduction mainly appears in the form of heat and electromagnetic waves. During bremsstrahlung, a continuous spectrum of photons with different energies is produced. The maximum energy in this spectrum is the maximum kinetic energy of the colliding electrons. This maximum energy is the potential applied to the tube. For example, if the potential is 90 kilovolts, then a spectrum of photons with energies ranging from 1 to 90 kiloelectron volts is formed. The mean energy of this spectrum is often one-third to half of the maximum energy. In our previous example, the mean energy is less than 40 KeV. There are numerous photons with low energies in the spectrum which are almost useless in imaging and only increase patient exposure, so a metallic plate, known as a filter, is used in the tube opening to absorb these photons. This filter is often made of beryllium. The figure below shows the spectrum with and without filtration.

Characteristic X-ray:

Electrons colliding with the target can also cause ionization. the approaching electron ejects the electron belonging to the outer layer of the atom and loses its energy during ionization or excitation. In this process, the vacant space of the ejected electron is filled with another electron from the outer layers and thus causes the emission of a photon mostly in form of X-rays. By taking the binding energy of the electron as the energy needed to separate the electron from the atom, and its difference in different atoms, this emission is an exclusive feature of every element. Therefore, it can be used to identify different elements thus it’s known as the characteristic X-ray. Characteristic X-rays appear as peaks in the spectrum of the X-ray beam.

X-ray beam spectrum of tungsten with characteristic X-rays at 90kVp

in a fixed current, increasing the voltage increases the height of both emission types meaning the number of photons has increased. Also, the bremsstrahlung peak is shifted towards higher energies meaning the mean spectrum energy has increased. This is while the position of characteristic X-rays is intact.

In a fixed voltage, increasing the current only increases the number of photons and the height of the spectrum, but the mean energy of the spectrum is untouched.

Increasing the voltage shifts the peak toward the higher energies.

Only a small percentage of the electron energy as small as 1 percent is converted into X-rays. The rest is converted into heat. The amount of heat produced limits the electron current and the intensity of X-rays. There are multiple ways to reduce heat, using rotating anodes is a common practice. A new way to reduce the heat is the utilization of a metallic current as the target.

For more information on the performance of X-ray tubes and the process of X-ray production, you can refer to the references below:

References:

• Tafti D, Maani CV. X-ray Production. [Updated 2022 Aug 1]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022  Jan-. https://www.ncbi.nlm.nih.gov/books/NBK537046
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