Beam Quality X-ray (with Half Value Layer)

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Beam Quality describes the shape of the energy spectrum (i.e. the energy distribution of the x-rays) and beam quantity describes the total intensity of the spectrum (i.e. the area under the x-ray spectrum curve).

Radiologic technologists directly need to take these factors into account when determining the technical factors to use for a given scan.

We describe the factors affecting beam quality including: kVp, target material, and pre-patient collimation. We also discuss the mA (tube current) which is the most important factor that affects only the beam quantity and not beam quality.

Why is there a correlation between beam quality and HVL?
The half value layer is indirectly measuring the beam quality since a harder beam will have a higher half value layer (HVL) and a softer beam will have a lower half value layer.

Harder x-ray spectrum will require more thickness of material as low energy x-rays are preferentially attenuated. Since a softer spectrum has more low energy photons, which get attenuated more easily, the half value layer will be lower for a soft spectrum.

An 80kVp beam (soft) and a 140kVp beam are filtered (shaded region in the figures represents spectrum after filtration) and a larger fraction of the photons are attenuated from a soft spectrum.

This correlation between beam quality and half value layer means that we can use half value layer as a surrogate or indirect measure of the beam quality. It is not a perfect metric but it does provide significant information about the spectrum and can be used for quality assurance trending. For instance if the HVL is suddenly reduced significantly on a system this means that there has been a change to x-ray spectrum which should be investigated.