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Scatter and collimation
Collimation shrinks the volume of tissue irradiated, cutting both scatter and patient dose.
The rule
Scatter is radiation that changes direction inside the patient, mostly through Compton interactions. It carries no useful information. When it reaches the receptor it adds fog, lowers contrast, and hides low-contrast detail. Three factors increase scatter: higher kVp, larger field size, and greater part thickness. Collimation is your most direct control over how much is produced.