To accomodate some data sets in which the resolution in the z-direction (in the direction of increasing slice number) is not equal to the within-slice resolution, provision is make for producing a new data set in which contiguous sets of n slices are merged into 1, producing a new data set having 1/n of the number of slice in the original set. Two algorithms (maximum value or majority value) are available for determining the assignment of values in each new slice.