Method for delineating a metal object for artifact reduction in tomography images, wherein - projection images of a body containing the metal object are received from a projection device, and - a volume model of at least part of the body is formed on the basis of the projection images, - voxels, which are part of a region encompassing the metal object and at least one metal artifact, are chosen in the volume model on the basis of a segmentation criterion and combined to form a first volume mask, - the first volume mask is projected onto the projection images and a respective projection mask is generated hereby, - each projection mask is adjusted on the basis of pixel values of the respective projection images, - a second volume mask is determined from the adjusted projection masks, - the second volume mask is adjusted on the basis of voxel values of the volume model, - the adjusted volume mask is supplied as a description of a three-dimensional form of the metal object, wherein the projection masks are each adjusted in that a region containing the projection mask is delineated in the respective projection image, which region is smaller than the overall projection image, but larger than the projection mask, and iteratively in at least one iteration in each case: overall, a metal allocation function of pixel values to the metal body and a background allocation function of pixel values to a metal-free image background is determined from pixel values of pixels of the region and for each pixel of the region the affiliation thereof to the projection mask (20, 20', 22) is defined again using its respective pixel value and on the basis of the two allocation functions (27, 28) and wherein the second volume mask is in each case adjusted in that in the volume model a region containing the second volume mask is delineated, which region is larger than the second volume mask and smaller than the overall volume model, and iteratively in at least one iteration in each case: overall, a volume-based metal allocation function of voxel values to the metal body and a volume-based background allocation function of voxel values to a metal-free volume background is determined from voxel values of voxels of the region and for each voxel of the region the affiliation thereof to the second volume mask (26', 26) is defined again using its respective voxel value and on the basis of the two allocation functions (29, 30). - projection images of a body containing the metal object are received from a projection device, and - a volume model of at least part of the body is formed on the basis of the projection images, - voxels, which are part of a region encompassing the metal object and at least one metal artifact, are chosen in the volume model on the basis of a segmentation criterion and combined to form a first volume mask, - the first volume mask is projected onto the projection images and a respective projection mask is generated hereby, - each projection mask is adjusted on the basis of pixel values of the respective projection images, - a second volume mask is determined from the adjusted projection masks, - the second volume mask is adjusted on the basis of voxel values of the volume model, - the adjusted volume mask is supplied as a description of a three-dimensional form of the metal object,