Ciassification of Artifacts in X-Ray Cr Images Due to Nonlinear Shadows

Polychromaticity and noise artifacts in computed x-ray tomography can be traced to a nonlinear dependence of projection data and its variance upon an ideal consistent set of projection data. We describe both types of artifacts and present mathematical models. A comparison is made between artifacts in the image i(r) of CT values and artifacts in the distribution ¿2(r) of the noise variance. Three classification schemes for these nonlinear artifacts are presented: the physical origin of the artifacts (I), the geometrical appearance of the artifact (II), and the degree of consistency of the nonlinear shadow (III).