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Beam Restriction
Collimate to the anatomy: less tissue irradiated means less scatter, better contrast, and lower patient dose.
The rule
Beam restriction limits the x-ray field to the anatomy of interest. A smaller field irradiates less tissue, so the patient receives less dose and the body produces less scatter. Less scatter reaching the detector means better image contrast. Few steps improve both dose and image quality at once, which is why collimation is expected on every exam.