A study of the image discrepancies due to object time-dependence in transmission and emission tomography

In conventional computed tomography (CT) imaging of a point object, projection filtering causes the back-projected contributions to image positions away from the point to sum to zero. If the point object intensity is time-dependent, and all the projections are not acquired simultaneously, this cancellation cannot be complete and artefacts result. Loss of spatial invariance makes a general linear-systems approach to the problem impossible. We have studied the properties of such artefacts by the computer simulation of decaying exponential time-dependence in three different spatial distributions and four transmission and emission CT geometries. Spatially complex time-dependent objects typically produce artefacts that can be treated as an additional broad-spectrum noise source with a power comparable to that of other CT noises. Artefacts from broad ranges of similar time-dependence can add coherently to cause patches of artefact, particularly in geometries with a strong correlation between projection acquisition time and projection angle. As expected, artefacts are reduced for all geometries as scan duration is reduced. In our model, with a most rapid decay constant of 1.2 min-1, negligible artefacts were observed for a six second scan duration.