1. A method for calculating the motion vector field (MVF) in a reconstructed Computed Tomography (CT) scan image, comprises: determining a region of interest (ROI) based on a vessel centreline in a first reconstructed CT scan image, wherein a motion vector field (MVF) for the ROI is to be calculated; determining time control points for calculating the MVF; calculating a motion level factor of the ROI; calculating motion amounts of the MVF corresponding to each of the time control points; determining the direction of the MVF of the ROI; calculating the MVF of the ROI according to the motion level factor, the motion amounts of the MVF corresponding to each of the time control points and the direction of the MVF; and reconstructing a target reconstructed image by performing a motion compensation on the first reconstructed CT scan image according to the MVF of the ROI.