1. A method for determining a displacement field of displacement vectors at a plurality of pixels in a sequence of images, the method comprising: receiving an input image sequence comprising a plurality of image frames individually including multidimensional image data corresponding to a plurality of pixel locations at different times; providing a fully constrained nonlinear equation set including: a plurality of displacement vector invariant equations, a forward displaced frame difference equation using as input data from only two of the image frames at one time t 1 and an other time t 2 , respectively, and a backward displaced frame difference equation using as input data from only the same two image frames; and using at least one processor, solving the equation set using an iteration equation and the image data of only the two image frames at times t 1 and t 2 to determine a displacement field describing displacement vectors at pixel locations at the one time by solving displacement field values at the other time from the plurality of displacement vector invariant equations.