Scatter, Beam Restriction and Grids

Where Scatter Comes From

Scatter is produced mainly by Compton interactions in the patient, where a photon ejects an outer-shell electron and continues in a new direction with less energy. Scatter leaves the patient at random angles and carries no useful information. At the digital detector they add unwanted signal that lowers image contrast. Three factors increase scatter: higher kVp, which makes Compton interactions more likely than photoelectric absorption; a larger field size; and greater part thickness, since both put more tissue in the beam. Scatter from the patient is also the main source of occupational dose for the radiographer.