Fig. 2.

This figure shows the same acquisition situation as Fig. 1. After acquisition of slices 15 (red slice) and 16 (green) the subject moved (“looked into the sky”) and slices 17 and 18 were acquired in the locations shown in yellow and blue respectively. We also assume that prior to slice 15 all slices were acquired with the subject in the same position and hence were parallel to slice 15 and correspondingly all slices after slice 18 were parallel to slice 18. The green parts of the volume have been acquired once, and once only, on a regular grid and standard interpolation would in principle be feasible. The yellow part has been acquired twice and there will be spots where voxel values from one slice will almost exactly coincide (spatially) with values from another slice and these values will have to be reconciled. Finally the red part has not been acquired at all which means that an interpolation will need to fill in values a long distance from the nearest observed values. For an interleaved acquisition, which is more typical, one would instead have multiple smaller yellow and red areas in close proximity, but the principle is the same.