Analytic determination of scatter exposure for radiography

The energy conversion probability from the primary X-ray to the first scattered ray in object material is analytically obtained in consideration of three interaction types, including photoelectric effect, Compton scatter and pair production. The approximative formulae for the scattering exposure and the minimal direct-scatter ratio on the imaging plane in radiography are given. The estimated minimal direct-scatter ratio for FTO is in agreement with the of National Insitute of Heath experiment result.