The present disclosure provides a method for processing projection data. The method may include obtaining a first image generated by performing a first scan to a subject by a first imaging device； determining first projection data based on the first image, the first projection data corresponding to a first area of the subject； obtaining second projection data by performing a second scan of the subject using a second imaging device, the second projection data corresponding to a second area of the subject, the first area at least partially overlapping with the second area in an overlapping area； determining registered first projection data by registering the first projection data to the second projection data with respect to the overlapping area； determining scatter component based on the registered first projection data and the second projection data, the scatter component including low-frequency scattered radiation signals.
