1. A method of segmenting an implant in a radiographic image of a portion of a patient's body that contains the implant, the method comprising: (a) smoothing the image by a lowpass first filter for reducing noise in order to obtain a smoothed image; (b) applying a non-directional spatial derivative second filter to the smoothed image in order to obtain a filtered image; (c) hysteresis thresholding pixels of the filtered image in order to obtain a thresholded image, the hysteresis thresholding comprising: (i) retaining pixels having a brightness value greater than or equal to a high first threshold, (ii) discarding pixels having a brightness value less than a low second threshold, and (iii) retaining pixels having a brightness value lying between the high first threshold and the low second threshold providing they are adjacent to at least one pixel having a brightness value greater than or equal to the high first threshold; (d) selecting an outline obtained in the thresholded image, thereby defining a segmented portion of the image corresponding to the implant; (e) modifying the low second threshold used during the hysteresis thresholding if the segmented portion obtained does not satisfy a calculated criterion, and reapplying the hysteresis thresholding and electing an outline in order to obtain a new segmented portion of the image; and if the calculated criterion is not satisfied after a predetermined number of modifications or after lowering the low second threshold to a minimum, then dilatations are performed followed by erosions of increasing size on the thresholded image until a closed outline is obtained defining the segmented portion, wherein prior to the smoothing at least one anisotropic filtering is applied to the image in order to decrease noise, and wherein the anisotropic filtering applies a filter to the preceding pixels of the image, which filter delivers at the following iteration for the pixel of coordinates s, a new pixel of coordinates s, defined by the equation: 
 u ( s,t+Δt )= u ( s,t )+Δ t *div ( f (∥∇ u ( s,t )∥)·∇ u ( s,t )) where: ∇ designates the gradient operator; div designates the divergence operator; f is a decreasing non-uniformity function; and Δt is a positive and non-zero predetermined increment.