A derivation procedure for a three-dimensional digital mask from a series of two-dimensional masks in a radiographic device containing a source (S) of X-rays, a means of recording and a volume of interest hat contains the object to be X-rayed located between the source (S) and the means of recording consists of an extrapolation of each mask Mθ2 includes determining a last segment 1fin beyond the limits of the means of recording; and working out a two-dimensional mask Mγ associated with a position Sγ of the source, for any angle γ included in the angular range θ2 to θ1 (a position close to θ2). For every parallel segment 1 located between segment d (or d′ respectively) and segment 1fin, the procedure further includes deriving a three-dimensional mask (21) of the object for each voxel at the intersection of plane P1θ2 and the volume of interest; and projecting the three-dimensional mask onto the segment 1.