1. A transformation method for diffusion spectrum imaging (DSI) comprising: a) receiving an original DSI dataset and a template DSI dataset; b) using a processor to compute an energy function E; c) using the processor to obtain, for each time point, a first-order derivative and a second-order derivative of the energy function E computed in step b) with respect to velocity field in an image space and a first-order derivative and a second-order derivative of the energy function E computed in step b) with respect to velocity field in a q-space; d) using the processor to compute, for each time point, the velocity field in the image space and the velocity field in the q-space based upon the first-order and second-order derivatives obtained in step c); e) using the processor to perform integration on the velocity fields obtained in step d) overtime to obtain a deformation field, which maps the original DSI dataset to the template DSI dataset, wherein the deformation field is calculated according to: ⅆ g 0 ⁢ t ⅆ t = v t ∘ g 0 ⁢ t ; and wherein the deformation field g 0t =(g x,0t , g q,0t ), where x is a three-dimensional coordinate in the image space, q is a three-dimensional coordinate in the q-space, g x,ot , which is an image space component of g 0t , is a function of x, and g q,0t , which is a q-space component of g 0t , is a function of both x and q; and the velocity field v t =(v x,t , v q,t ) where v x,t , which is an image space component of v t , is a function of x, and v q,t , which is a q-space component of v t , is a function of both x and q; and f) using the processor to generate a transformed DSI dataset according to the deformation field obtained in step e).