What factors affect the X-ray spectrum?

The X-ray spectrum describes the distribution of photon energies produced by an X-ray tube. Its shape and energy distribution are influenced by several factors, most importantly tube voltage (kVp), beam filtration, target material, and the generator waveform (voltage ripple).

The X-ray spectrum is determined by tube voltage, filtration, target material, and generator waveform, which together control the energy distribution and intensity of the X-ray beam.

These factors determine the maximum photon energy, the average photon energy, and the relative number of photons produced at different energies, which ultimately influences beam penetration, image contrast, and radiation dose.

Understanding the physics

The X-ray spectrum generated in an X-ray tube consists primarily of a continuous Bremsstrahlung distribution with superimposed characteristic radiation peaks. The overall shape of this spectrum depends on how electrons interact with the target material and how the beam is modified before leaving the tube.

The tube voltage (kVp) is the most important factor affecting the spectrum. The maximum photon energy produced cannot exceed the kinetic energy of the incident electrons, which is determined by the applied voltage:

Increasing kVp therefore extends the spectrum to higher energies and increases the number of Bremsstrahlung photons produced. Higher electron energies also increase X-ray production efficiency, resulting in a greater overall photon output.

Beam filtration also alters the spectrum by preferentially removing low-energy photons. As the X-ray beam passes through filter material such as aluminium, low-energy photons are more likely to be absorbed through the photoelectric effect. This reduces the number of low-energy photons in the beam while increasing the average photon energy, a process known as beam hardening.

The target material of the anode influences the spectrum in two ways. First, it determines the energies of characteristic radiation peaks, which occur at photon energies corresponding to the binding energies of the target atom’s electron shells. Tungsten produces characteristic peaks at approximately 59 keV and 67 keV, which appear superimposed on the continuous Bremsstrahlung spectrum. Second, the atomic number (Z) of the target affects the overall Bremsstrahlung production efficiency. Higher atomic number materials generate more Bremsstrahlung radiation, increasing the intensity of the spectrum. This is one reason tungsten (Z = 74) is widely used in diagnostic X-ray tubes.

Another important factor is the generator waveform, which determines how constant the tube voltage remains during an exposure. In generators with significant voltage ripple, such as older single-phase systems, the tube voltage fluctuates during the exposure cycle. Because electron energy varies with voltage, this produces a spectrum that contains more lower-energy photons. Modern high-frequency generators produce a much more constant voltage with minimal ripple, resulting in a spectrum with a higher effective photon energy and greater output efficiency.

Together, these factors determine the energy distribution of photons produced by the X-ray tube.

Where this matters clinically

Changes in the X-ray spectrum influence both image quality and patient radiation dose. Increasing kVp produces a more penetrating beam with higher-energy photons, while filtration removes photons that would otherwise be absorbed by the patient without contributing to image formation.

Generator design and target material also influence the efficiency and energy distribution of the beam, which is why modern X-ray systems use high-frequency generators and high atomic number target materials such as tungsten.

Understanding how these factors shape the X-ray spectrum helps explain how exposure parameters control beam penetration, image contrast, and radiation dose.