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.
For our detailed post on beam quality please see: howradiologyworks.com/beamqua...
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.