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 encompassing only a subset of the diffusion weighted image data; a calculation unit configured to calculate at least one of diffusion coefficient and fractional anisotropy data serving as an index of diffusion anisotropy within 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 data calculated by said calculation unit.