1. An image processing apparatus comprising: at least one computer connected to communicate with memory storing (a) a plurality of magnetic resonance images acquired from a subject representing anatomical structures having fluid portions therewithin and (b) executable computer program instructions which, when executed by said at least one computer; specify a fluid region which is a region corresponding to a fluid portion of a subject in a plurality of magnetic resonance images that were acquired by applying a labeling pulse to a label region and that are mutually related; derive a plurality of index values indicating dynamics of a fluid based on the specified fluid region in the images and derive a velocity of the fluid based on the index values, wherein the dynamics include the fluid repeatedly moving forward and backward or the fluid repeatedly diffusing and contracting during the time data defining the images was acquired; and display a graph plotting the index values against time and the derived velocity, wherein each index value of the graph indicates the dynamics of the fluid repeatedly moving forward and backward or the dynamics of the fluid repeatedly diffusing and contracting.