D

Using an open-ended, shielded (rectangular) cone sharply limits the beam size; this alone cuts the patient’s absorbed dose by about four-fold—more than the 50 % reduction from faster (E-speed) film and far exceeding the benefit of added filtration or lead aprons, which mainly shield organs outside the beam [Rohlin & White, 1992, PMID 1520917; Horner & Hirschmann, 1990, PMID 2212199].