X-Ray Energy and Spectra Filters

X-ray Energy Spectra and Filters

A key difference between gamma rays and X-rays is the energy of the beam. Typically, a gammaray source will have just one energy: this is known as a monoenergetic beam. X-rays, on the otherhand, are a large combination of many energies known as a polyenergetic beam. Thecombinations of these energies make up what’s called the beam’s X-ray spectrum. Scientistsmeasure the energy of photons in the spectrum in electron volts (eV); radiobiology studiestypically employ photons in the kilo-electron volt (keV) range, while clinical treatments typicallyemploy photons in the mega-electron (MeV) range. These energies refer to the potential appliedto the electrons hitting the target in the X-ray tube – which is how the X-rays are produced. Incontrast, gamma rays are produced from a radioactive material.

An example spectrum of a 320 kVp beam (meaning its peak voltage is 320kV, producing 320 keVphotons), is shown below:

The X-ray spectrum of a beam is very important because of how X-rays are absorbed. Lowerenergy X-rays are absorbed very easily, even by the skin. High energy X-rays, on the other hand,can penetrate further into tissue. As a result, the higher the average energy of the beam, thebetter it penetrates tissue. A comparatively higher average energy beam is said to be a harder beam.

To set the X-ray spectrum, there are two parameters that can be adjusted:

1) The maximum energy, or kVp of the beam

2) The added filtration placed in front of the beam. This is typically thin pieces of metal suchas aluminum, copper or tin.

Adding more filtration to a beam is known as hardening it. From this information, it is naturalthat different X-ray spectra are used for different applications. Below are some examples:

Small animal and cell radiation:

  • A 320kVp beam with a Thoraeus filter (Al, Cu, Sn) used for bone marrow ablation.
  • A 225 kVp beam with a Cu filter used for targeted irradiation.
  • A 160 kVp beam with an Al filter used for irradiating cells.
  • A 40 kVp beam with an Al filter used for soft-tissue imaging.

Human radiation:

  • A 6 MV (megavolt) beam with a flattening filter used for treating humans.
  • A 120 kVp beam with an aluminum filter used for imaging humans.

References:

1. Poludniowski, G., Landry, G., DeBlois, F., Evans, P. M. & Verhaegen, F. SpekCalc: a program tocalculate photon spectra from tungsten anode X-ray tubes. Phys Med Biol 54, N433-438 (2009).

2. American Cancer Society: What Are X-rays and Gamma Rays?https://www.cancer.org/cancer/risk-prevention/radiation-exposure/X-rays-gamma-rays/what-are-xrays-and-gamma-rays.html.

3. NIH Science Education: X-rays. https://www.nibib.nih.gov/science-education/science-topics/X-rays

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