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; and f) using the processor to generate a transformed DSI dataset according to the deformation field obtained in step e).