Scatter radiation intensities about mammography units
Abstract
The intensity of scattered x rays about two clinically installed mammographic units was measured at 25, 30, and 35 kVp over a range of scattering angles. The scatter-to-primary ratio for backscattering is measured to be 4 to 7 times greater than that for 90 degrees scattering. The scatter fraction is the ratio of scatter dose at 1 m to primary dose at 1 m per primary x-ray field area. Averaged over various phantom and field sizes, the scatter fraction for mammography ranges from 0.01 x 10(-6) cm-2 to 0.2 x 10(-6) cm-2 at 0 degrees, approximately 0.25 x 10(-6) cm-2 at 90 degrees, to 1.7 x 10(-6) cm-2 at 163 degrees, with not more than 30% difference between the scatter intensity generated at 25 and 35 kVp. The differences in the scatter fraction measured off two mammography units from two manufacturers are less than 10% for scattering angles greater than 45 degrees. However, the intensity of forward scatter depends strongly on the construction of the image receptor assembly and x-ray tube collimator, with a sharp rise in the scatter fraction at approximately 25 degrees for the 18 x 24 cm2 image receptor apparently due to scatter off the collimator. The dose (normalized at 1 m per 100 patients) in directions toward adjacent walls, ceiling, and floor is conservatively calculated, providing a quick method for estimating the unshielded dose to an occupied area.
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