1. A method for filtering a plurality of 2-D x-ray projection images of a subject for use in a 3-D image reconstruction, the method comprising: processing each of the plurality of 2-D x-ray project images by, for each pixel of a plurality of pixels in each 2-D projection image: computing a variable weight value according to the distance between each pixel and a central pixel of the 2-D projection image upon which a central x-ray has been incident; and applying a diffusion process for each pixel with a fixed weight value for in-plane adjacent pixels and the variable weight value for out-of-plane adjacent pixels; reconstructing a diagnostic 3-D image from the plurality of processed 2-D x-ray projection images; and transmitting, storing, or displaying the reconstructed diagnostic 3-D image.