14. Slice Spacing

For most datasets slice-spacing is be consistent, and setting the Slices/mm value in the Settings tab of the Output panel is sufficient. In some datasets however slice-spacing is variable, either as a result of errors in data collection, or a change in collection parameters at some point in the sequence. While correct slice spacings are not always known in these circumstances, educated guesses at them can usually be made that will improve model results.

The reconstruction approach used by SPIERS theoretically assumes consistent splice spacing, but is flexible enough to be able modify models to be tweaked in SPIERSview after construction to allow for some inconsistency in slice spacing. To facilitate this SPIERSedit provides a mechanism for modifying slice spacing. This should not be used as a replacement for a correct Slices/mm setting however; to ensure the best reconstruction this value should be set to something close to the average spacing for all slices before exact slice positions are subsequently tweaked.

To alter slice spacing, the Show position in slice selector item in the Slice Spacing menu should be selected. With this option turned on, the filenames in the Slice Selector panel are replaced with positions (in mm) of the slice, relative to a 0 position inferred for notional slice 0 (so if Slices/mm is set to 10, slice 1 will be at 0.1mm, slice 2 at 0.2mm etc).

Individual slice positions can be set by selecting the slice and using the Set slice position command on the Slice Spacing menu. SPIERSedit will not allow a user to set the position in front of the next slice or behind the preceding one; if this is attempted the program will warn the user and move the positions of other slices to maintain slice order.

If several slices share a consistent spacing, this can be set using the Change slice spacing command on the Slice Spacing menu. Select slices to be affected then use the command – slice spacing can be set as an absolute value or a percentage of standard slice spacing (the Slices/mm setting).

We stress again that the mechanism to deal with inconsistent slice spacing in SPIERS is something of a bodge, and while it will improve the reconstruction of inconsistently spaced data, the process of reconstruction (using SPIERS or any other software) is eased by the production of consistently spaced data in the first place.