  Metal streak artifacts in X-ray computerized tomography (CT) are rigorously
characterized here using the notion of the wavefront set from microlocal
analysis. The metal artifacts are caused mainly from the mismatch of the
forward model of the filtered back-projection; the presence of metallic
subjects in an imaging subject violates the model's assumption of the CT
sinogram data being the Radon transform of an image. The increasing use of
metallic implants has increased demand for the reduction of metal artifacts in
the field of dental and medical radiography. However, it is a challenging issue
due to the serious difficulties in analyzing the X-ray data, which depends
nonlinearly on the distribution of the metallic subject. In this paper, we
found that the metal streaking artifacts cause mainly from the boundary
geometry of the metal region. The metal streaking artifacts are produced only
when the wavefront set of the Radon transform of the characteristic function of
a metal region does not contain the wavefront set of the square of the Radon
transform. We also found a sufficient condition for the non-existence of the
metal streak artifacts.
