Abstract: 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 M?2 includes determining a last segment lfin beyond the limits of the means of recording; and working out a two-dimensional mask M? associated with a position S? of the source, for any angle ? included in the angular range ?2 to ?1 (a position close to ?2). For every parallel segment l located between segment d (or d? respectively) and segment lfin, the procedure further includes deriving a three-dimensional mask (21) of the object for each voxel at the intersection of plane P1?2 and the volume of interest; and projecting the three-dimensional mask onto the segment l.