B

Filtration (e.g., aluminum filters) preferentially absorbs low-energy, long-wavelength x-ray photons that would be completely absorbed in the patient and add dose without improving the image; removing these photons “hardens” the beam and lowers unnecessary exposure [MacDonald-Jankowski & Lawinski, 1992, PMID 1450837; Al Khalifah et al., 2020, PMID 31749130].