1. A method for estimating a velocity field using an image sequence having more than two frames, the method comprising: providing a set of equations including an integer number M−1 displaced frame difference equations, where M is greater than 2; receiving an input image sequence comprising at least M image frames individually including multidimensional image data corresponding to a plurality of pixel locations at M different times; and using at least one processor, solving the equation set using a set of iteration equations and the image data of the M image frames to determine a velocity field describing velocity vectors at pixel locations at a first one of the M different times, wherein the displaced frame difference equations are given by: 
 DFD ijs =I ( i +( t s −t 1 ) u ij ,j +( t s −t 1 ) v ij ,t s )− I ij ( t 1 )=0∀(1 <s≦M ). wherein I is an intensity of the image data, i is a pixel index in a first direction x, j is a pixel index in a second direction y orthogonal to the first direction x, s is a frame index, t s is a time corresponding to a currently indexed frame, t 1 is the first one of the M different times, u ij is a first component of the velocity field along the first direction x, and v ij is a second component of the velocity field along the second direction y.