1. A magnetic resonance (MR) imaging apparatus comprising: an MRI system having components including static and gradient magnetic field generators, at least one radio frequency (RF) coil coupled to an imaging volume, at least one RF transmitter, at least one RF receiver and at least one control computer having a processor and memory configured to control said system components so as to effect: repeated acquisition of MR image data in a synchronized manner with a trigger signal using a phase-shift imaging process, the phase-shift imaging method including (a) application of an excitation pulse to an imaging region including a Cerebrospinal Fluid (CSF), (b) reconstructing one image based on a group of echo signals collected by applying one excitation pulse, and (c) applying a flow encode gradient magnetic field to give a phase shift proportional to a velocity onto the reconstructed image, thereby obtaining velocity components of the CSF; creation of corresponding plural velocity images, a velocity image being created for each repeated trigger signal and indicating a distribution of velocity components; creation of an average velocity image based on the plurality of reconstructed images; and calculation of variances of the velocity components, with respect to each of same positions on the plurality of reconstructed images, based on the plurality of reconstructed velocity images and the average velocity image.