Artefacts in spiral-CT images and their relation to pitch and subject morphology.

This qualitative study is intended to create awareness of artefacts that are associated with spiral-CT imaging. A simple description of spiral-CT reconstruction is used to explain how these artefacts depend on the pitch and subject morphology, and shows when these artefacts are likely to impair the diagnostic value of the acquired images. We scanned a cone and rod phantom with pitch 2, and used the acquired images to demonstrate how spiral data acquisition and interpolation leads to artefacts in the reconstructed images. We then demonstrated the effects of various pitches in scans of a human cadaver, whereas the slice thickness was kept constant. Some patient studies are presented in order to show the possible clinical consequences. Spiral acquisition may cause geometric distortions and apparent inhomogeneity of homogeneous structures. We were able to link these artefacts to the way in the acquisitions were done, and the reconstructions were performed. We have shown how these artefacts can be anticipated in clinical studies. When areas of low contrast, surrounded by hypo- or hyperdense structures, are scanned with a large pitch and viewed with a narrow window, spiral artefacts may influence the diagnostic quality of the images. These effects should be considered when choosing the pitch.