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Filtration
Filtration strips useless low-energy photons, hardening the beam and cutting skin dose.
The rule
An x-ray beam leaves the target as a mix of photon energies. The lowest-energy photons have almost no chance of passing through the patient to reach the receptor. They are absorbed in the first few centimeters of tissue, adding to skin dose and contributing nothing to the image. Filtration removes them before they reach the patient. Its main purpose is reducing patient dose, especially entrance skin dose.