Aliasing: A Source of Streaks in Computed Tomograms

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In computed tomography, each projection, or view, is broken up into n data samples. If the interval between samples is d there is an associated Nyquist frequency KN = 1/(2d), which represents the highest spatial frequency retrievable after the sampling process. If higher frequencies are present, as in the case of sharp edges of bone, they will cause aliasing errors which show up on the final image as streaks radiating from the edges. These streaks differ from interpolation streaks in that they are not synchronous with the view angles and are not affected by the number of views. Three remedies for aliasing streaks are (i) broadening the source-detector aperture function so as to physically filter out the high frequencies before sampling: (ii) finer sampling of the data, followed by digital filtering to remove the high frequencies (attractive with translate-rotate or stationary detector scanners); or (iii) offsetting the detector sampling points (attractive with rotating-detector 360° scanners). With remedy iii. opposing views can either be interleaved, to double the spatial resolution, or processed separately, in which case the aliasing error cancels. These remedies are illustrated for both parallel and fan-beam projections. In the case of fan beam projections, the cancellation is close but not exact, because opposing rays do not precisely coincide.

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