1. An image processing apparatus comprising: a storage unit configured to store a plurality of volume data acquired by imaging a predetermined part of an object, the plurality of volume data corresponding to a plurality of phases; a calculation unit configured to calculate a spatial motion vector of each voxel included in each volume data by performing registration between the plurality of volume data; an image generation unit configured to generate an image representing a local motion of the diagnosis part using the motion vector of each voxel; and a display unit configured to display the image representing the local motion of the diagnosis part.