Your question reminded me of an old thread on how to divide up a region based on distance.
https://afni.nimh.nih.gov/afni/community/board/read.php?1,157456,157474#msg-157474
A couple small corrections - from 3dcalc's help, the use of the 'l' ("ell") character is for the 4th dimension index. The RAI order is *NOT* assumed unless -RAI or -dicom is given as an option; the storage orientation is assumed for xyz.
Similarly, the '-i', '-j', and '-k' values, if not otherwise used,
will be loaded with the voxel spatial index coordinates. The '-l'
(letter 'ell') value will be loaded with the temporal index coordinate.
The '-n' value, if not otherwise used, will be loaded with the overall
voxel 1D index. For a 3D dataset, n = i + j*NX + k*NX*NY, where
NX, NY, NZ are the array dimensions of the 3D grid. [29 Jul 2010]
Otherwise undefined letters will be set to zero. In the future, new
default values for other letters may be added.
NOTE WELL: By default, the coordinate order of (x,y,z) is the order in
********* which the data array is stored on disk; this order is output
by 3dinfo. The options below control can change this order:
-dicom }= Sets the coordinates to appear in DICOM standard (RAI) order,
-RAI }= (the AFNI standard), so that -x=Right, -y=Anterior , -z=Inferior,
+x=Left , +y=Posterior, +z=Superior.
-SPM }= Sets the coordinates to appear in SPM (LPI) order,
-LPI }= so that -x=Left , -y=Posterior, -z=Inferior,
+x=Right, +y=Anterior , +z=Superior.
