A computer is supplied with at least three 2-D projections of a subject that are offset from one another by difference angles. A position in the projection surface and a respective surface data value per projection are allocated to the respective surface elements of the projection surface. On the basis of the supplied projections, the computer determines binary volume data values for a number of volume elements, a position in space being allocated to the volume elements, so that their totality represents a 3-D image of the subject.