"HVL is particularly useful for characterising polyenergetic x-ray beams (such as those used in diagnostic radiology), where individual linear attenuation coefficients (μ) vary with energy. Unlike linear and mass attenuation coefficients, which are strictly energy-dependent and ideal for monoenergetic beams, HVL provides a practical summary of a beam’s overall penetrability."
"HVL serves as a surrogate measure of beam quality or x-ray spectrum penetrability. A lower HVL suggests a beam with a lower mean photon energy and greater attenuation per unit thickness. HVL is conventionally measured in millimetres of aluminium for diagnostic x-rays."
"As an x-ray beam passes through a filter or tissue, beam hardening occurs: lower-energy photons are preferentially absorbed via the photoelectric effect, increasing the average energy of the remaining photons. As a result, each subsequent HVL is larger than the first (1st HVL < 2nd HVL < 3rd HVL), since higher-energy photons require thicker material to be attenuated by half."
"As an x-ray beam passes through a filter or tissue, beam hardening occurs: lower-energy photons are preferentially absorbed via the photoelectric effect, increasing the average energy of the remaining photons. As a result, each subsequent HVL is larger than the first (1st HVL < 2nd HVL < 3rd HVL), since higher-energy photons require thicker material to be attenuated by half."
"Monoenergetic beamsFor monoenergetic beams HVL is directly related to linear attenuation coefficient (µ):HVL = 0.693 / μ."
"Polyenergetic beamsPolyenergetic beams have photons of varying energies. Unlike monoenergetic beams, polyenergetic beams undergo beam hardening, as the low energy photons are preferentially attenuated. Thus each subsequent HVL is different than the previous."
"Because of this, the HVL is determined experimentally rather than through a simple formula. The attenuation curve is measured by passing the x-ray beam through increasing thicknesses of material, plotting intensity vs. thickness, and finding the point where intensity drops to 50%."
"Beer-Lambert law"
Expected headings
"Calculation"
"Application"
"Beam hardening"