1. An image processing apparatus comprising: a storage unit configured to store diffusion weighted image data; a specifying unit configured to specify a calculation target region on the diffusion weighted image data; a calculation unit configured to calculate at least one of a diffusion coefficient and a fractional anisotropy serving an index of diffusion anisotropy with regard to the calculation target region based on the diffusion weighted image data; and a display unit configured to display at least one of the diffusion coefficient and the fractional anisotropy calculated by said calculation unit.