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Added vs Inherent Filtration
Total filtration = inherent + added. Above 70 kVp it must be at least 2.5 mm aluminum equivalent.
The rule
Total filtration is the sum of inherent filtration and added filtration. Inherent filtration is built into the x-ray tube itself. Added filtration is everything placed in the beam after the tube window. Filtration is expressed in millimeters of aluminum equivalent, meaning the thickness of aluminum that would attenuate the beam the same amount.