1. An image processing apparatus comprising: a displacement vector generator configured to detect a reference voxel of reference volume data and a corresponding target voxel of target volume data that have been acquired at intervals of a predetermined time period, and to generate a displacement vector that relates to the detected reference voxel and target voxel; and an intermediate volume data generator configured to generate at least one piece of intermediate volume data with respect to the reference volume data and the target volume data by using the generated displacement vector.