Usage: 3dLFCD [options] dset
Computes voxelwise local functional connectivity density as defined in:
Tomasi, D and Volkow, PNAS, May 2010, 107 (21) 9885-9890;
DOI: 10.1073/pnas.1001414107
The results are stored in a new 3D bucket dataset
as floats to preserve
their values. Local functional connectivity density (LFCD; as opposed to global
functional connectivity density, see 3dDegreeCentrality), reflects
the extent of the correlation of a voxel within its locally connected cluster.
Conceptually the process involves:
1. Calculating the correlation between voxel time series for
every pair of voxels in the brain (as determined by masking)
2. Applying a threshold to the resulting correlations to exclude
those that might have arisen by chance
3. Find the cluster of above-threshold voxels that are spatially
connected to the target voxel.
4. Count the number of voxels in the local cluster.
Practically the algorithm is ordered differently to optimize for
computational time and memory usage.
The procedure described in the paper defines a voxels
neighborhood to be the 6 voxels with which it shares a face.
This definition can be changed to include edge and corner
voxels using the -neighborhood flags below.
LFCD is a localized variant of binarized degree centrality,
the weighted alternative is calculated by changing step 4
above to calculate the sum of the correlation coefficients
between the seed region and the neighbors. 3dLFCD outputs
both of these values (in separate briks), since they are
so easy to calculate in tandem.
You might prefer to calculate this on your data after
spatial normalization, so that the range of values are
consistent between datasets. Similarly the same brain mask
should be used for all datasets that will be directly compared.
The original paper used a correlation threshold = 0.6 and
excluded all voxels with tSNR < 50. 3dLFCD does not discard
voxels based on tSNR, this would need to be done beforehand.
Options:
-pearson = Correlation is the normal Pearson (product moment)
correlation coefficient [default].
-spearman AND -quadrant are disabled at this time :-(
-thresh r = exclude correlations <= r from calculations
-faces = define neighborhood to include face touching
edges (default)
-faces_edges = define neighborhood to include face and
edge touching voxels
-faces_edges_corners = define neighborhood to include face,
edge, and corner touching voxels
-polort m = Remove polynomial trend of order 'm', for m=-1..3.
[default is m=1; removal is by least squares].
Using m=-1 means no detrending; this is only useful
for data/information that has been pre-processed.
-autoclip = Clip off low-intensity regions in the dataset,
-automask = so that the correlation is only computed between
high-intensity (presumably brain) voxels. The
mask is determined the same way that 3dAutomask works.
This is done automatically if no mask is proveded.
-mask mmm = Mask to define 'in-brain' voxels. Reducing the number
the number of voxels included in the calculation will
significantly speedup the calculation. Consider using
a mask to constrain the calculations to the grey matter
rather than the whole brain. This is also preferable
to using -autoclip or -automask.
-prefix p = Save output into dataset with prefix 'p', this file will
contain bricks for both 'weighted' and 'binarized' lFCD
[default prefix is 'LFCD'].
Notes:
* The output dataset is a bucket type of floats.
* The program prints out an estimate of its memory used
when it ends. It also prints out a progress 'meter'
to keep you pacified.
-- RWCox - 31 Jan 2002 and 16 Jul 2010
-- Cameron Craddock - 13 Nov 2015
=========================================================================
* This binary version of 3dLFCD is compiled using OpenMP, a semi-
automatic parallelizer software toolkit, which splits the work across
multiple CPUs/cores on the same shared memory computer.
* OpenMP is NOT like MPI -- it does not work with CPUs connected only
by a network (e.g., OpenMP doesn't work across cluster nodes).
* For some implementation and compilation details, please see
https://afni.nimh.nih.gov/pub/dist/doc/misc/OpenMP.html
* The number of CPU threads used will default to the maximum number on
your system. You can control this value by setting environment variable
OMP_NUM_THREADS to some smaller value (including 1).
* Un-setting OMP_NUM_THREADS resets OpenMP back to its default state of
using all CPUs available.
++ However, on some systems, it seems to be necessary to set variable
OMP_NUM_THREADS explicitly, or you only get one CPU.
++ On other systems with many CPUS, you probably want to limit the CPU
count, since using more than (say) 16 threads is probably useless.
* You must set OMP_NUM_THREADS in the shell BEFORE running the program,
since OpenMP queries this variable BEFORE the program actually starts.
++ You can't usefully set this variable in your ~/.afnirc file or on the
command line with the '-D' option.
* How many threads are useful? That varies with the program, and how well
it was coded. You'll have to experiment on your own systems!
* The number of CPUs on this particular computer system is ...... 1.
* The maximum number of CPUs that will be used is now set to .... 1.
=========================================================================
++ Compile date = Oct 31 2024 {AFNI_24.3.06:linux_ubuntu_24_64}
