1. A method implemented on a computing device having at least one processor and at least one computer-readable storage medium, the method comprising: receiving a first set of raw data relating to a subject generated by an imaging device; determining a first ROI in a field of view (FOV) of the imaging device; determining a second ROI including the first ROI; reconstructing the first set of raw data to obtain a first image on the second ROI; obtaining a second image by zero padding the first ROI of the first image; performing forward projection on the first image to obtain a first sinogram; performing forward projection on the second image to obtain a second sinogram; applying a first filter on the first sinogram to obtain a third sinogram; subtracting the second sinogram from the third sinogram to obtain a fourth sinogram; applying a second filter on the first set of raw data to obtain a fifth sinogram; and performing, based on the fourth sinogram and the fifth sinogram, iterative reconstruction to generate a third image.