An image processing apparatus is provided. The apparatus includes a setting unit configured to set a radiation exposure condition; a radiation generation unit configured to generate a radiation beam according to the radiation exposure condition; a two-dimensional radiation sensor configured to transform the reached radiation beam into a radiation image data and output the radiation image data; a storage unit configured to store the radiation image data; a detecting unit configured to detect a scattered radiation fluctuation in the reached radiation beam based on a comparison of two radiation image data with different radiation exposure condition, where the two radiation image data are selected within the plurality of radiation image data stored in the storage unit; and an image processing unit configured to extract a outline of a region of interest from the radiation image data based on the scattered radiation fluctuation detected by the detecting unit.