Projection Choice and Organ Dose

Entrance vs exit

The x-ray beam weakens as it crosses the body, because tissue absorbs and scatters it. The entrance skin dose, where the beam enters, is the highest dose in the field. Through an adult abdomen or lumbar spine, only about 1 percent or less of the entrance dose reaches the exit surface, so the entrance dose can be 100 times the exit dose or more. Even the air-filled chest lets only a small fraction through. An organ near the entrance surface therefore absorbs far more dose than the same organ near the exit surface, even though both are inside the field.