CT FILTRATION ALIASING ARTIFACTS

In a continuous implementation of filtered backprojection for computerized tomography (CT), projections are filtered prior to backprojection. The filter has a frequency response that is given by the absolute value of the frequency component. When backprojection is implemented in a discrete environment, the filtration operation can be derived by sampling the filter either in the Fourier or spatial domains. It is shown that the Fourier domain version leads to a dc shift and some low frequency shading in the resulting reconstructions because of aliasing of the filter. The primary purpose of this paper is to show that the aliasing occurs because the inverse Fourier transform of a band-limited version of the continuous filter has infinite spatial extent. A method is also proposed to reduce the aliasing artifacts.