In an image of lithiasis in a computed tomography examination, a segment of the image is chosen with a detection of the maximum of the gradient of radiological density measured in the voxels of the reconstructed image. This mode of action gives an evaluation of the envelope of the lithiasis that is objective and independent of the practitioner who performs it. Furthermore, to make the characterization of the lithiasis more realistic, especially to follow the effects of a therapeutic treatment, a characteristic result is given, equivalent to the volume of the lithiasis, preferably a weighted volume, and to the mean density of the lithiasis. The disclosed method also can facilitate the scheduling of surgical operations when it is not possible to administer oral treatment or carry out lithotripsy.