What determines X-ray beam quality?

X-ray beam quality refers to the penetrating ability of the X-ray beam, which is determined by the energy of the photons within the beam. A higher-quality beam contains higher-energy photons and is therefore more penetrating.

X-ray beam quality describes the penetrating power of the beam and is primarily determined by kVp and filtration.

The two main factors that determine beam quality are tube voltage (kVp) and filtration. Increasing kVp raises the maximum and average photon energies in the spectrum, while filtration removes low-energy photons, increasing the average energy of the beam.

Understanding the physics

The energy distribution of photons in an X-ray beam is described by the X-ray spectrum. The maximum photon energy in the spectrum is determined by the tube voltage:

However, the maximum energy alone does not fully describe beam quality. Most photons produced in the X-ray tube have energies lower than this maximum value, so the average photon energy provides a better measure of beam penetration.

In diagnostic radiology, the average photon energy is typically around one-third to one-half of the maximum photon energy:

Increasing kVp shifts the entire spectrum toward higher photon energies. This increases the number of high-energy photons capable of penetrating the patient and reaching the detector.

Filtration also affects beam quality. As the X-ray beam passes through materials such as the tube housing, window, and added filters, low-energy photons are preferentially absorbed. Because these photons contribute little to image formation and mainly increase patient dose, their removal increases the average energy of the remaining beam, a process known as beam hardening.

Beam quality is often assessed using the half-value layer (HVL), which represents the thickness of material required to reduce the beam intensity by half.

Where this matters clinically

Beam quality has important implications for both image quality and radiation dose. A higher-quality beam penetrates the patient more effectively and is less likely to be absorbed in superficial tissues, which can reduce unnecessary patient dose.

However, increasing beam quality by raising kVp also increases the proportion of scatter radiation produced, which can reduce image contrast. Radiographic technique therefore involves balancing beam penetration with image contrast.

Understanding beam quality helps explain why different imaging examinations use different kVp settings and why filtration is used to optimise the X-ray beam.