fScan Reference Manual, Chapter 2 (CmdLineOptions:Expr): segment
usage: -expr segment [flag value]
The SEGMENT command process a 3-D image map looking for voxel values greater than or equal to VALUE. Each time it finds such a voxel it uses a variation of the flood function to find all other voxels greater than VALUE contiguous with the first, and assigns all those values a new “cluster” number. Clusters can have as few as 1 voxel. Cluster values start at 1, and increase by 1 for each new cluster found.
VALUE: default is 1
0 - default
1 - also look for negative voxel values, less than or equal to -VALUE (default is just positive)