Differences between volumes can be assessed by subtracting one volume from the other; the result is a difference map. Such differences provide one way to identify or isolate small masses in 3D reconstructions. The statistics of the volumes may be used to determine the significance of 3-dimensional features (see pages 252-254 in Three-Dimensional Electron Microscopy of Macromolecular Assemblies by J. Frank).

Before subtracting volumes, make sure they are properly aligned.

• Check alignment of two volumes. shiftvol.spi uses CC and PK 3 to determine if volumes are shifted relative to each other.
• Ligands can be isolated by creating a difference map. diffmap.spi computes difference bewteen two volumes.
• Alternatively, the SPIDER operation DR DIFF computes the error and difference between two volumes.
• For display and illustration purposes, this recipe of interactive operations may be entered in SPIDER to extract and isolate a mass from a volume:

Source: docs/techs/diffmaps/diffmaps.html     Page updated: 09/15/06