Calculate Framewise Displacement (FD)
Arguments
X |
An N by 6 matrix in which the first three columns represent the
translational RPs (trans_units ), and the second three columns represent
the rotational RPs (rot_units ). If rot_units measures an angle,
it will be converted to trans_units by measuring displacement on a
sphere of radius brain_radius trans_units .
Alternatively, this can be the file path to an N by 6 matrix which can be
read with read.table (fields separated by white-space; no
header). |
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trans_units |
"mm" for millimeters (default), "cm"
for centimeters, or "in" for inches.
|
---|
rot_units |
"deg" for degrees (default), "rad" for radians,
or one of the trans_units options.
|
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brain_radius |
If rot_units measures an angle, the rotational RPs
are transformed to a spatial measurement representing the displacement on a
sphere of radius brain_radius trans_units .
If brain_radius is NULL (default), its value will be set to
(the equivalent of) 50 mm. |
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detrend |
Detrend each RP with the DCT before computing FD?
Default: FALSE . Can be a number of DCT bases to use, or TRUE
to use 4. |
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cutoff |
FD values higher than this will be flagged. Default: .3 . |
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Value
A list with components
- measure
A length N vector of FD values in trans_units
.
- measure_info
"FD"
- outlier_cutoff
cutoff
- outlier_flag
A length-N logical vector, where TRUE
indicates suspected outlier presence.
Details
The FD formula is taken from Power et. al. (2012):
FDi=|Δxi|+|Δyi|+|Δzi|+|Δαi|+|Δβi|+|Δγi|
where i is the timepoint; x, y and z are the
translational realignment parameters (RPs);
α, β and γ are the rotational RPs;
and Δxi=xi−1−xi (and similarly for the other RPs).
References
Power, J. D., Barnes, K. A., Snyder, A. Z., Schlaggar, B. L. & Petersen, S. E. Spurious but systematic correlations in functional connectivity MRI networks arise from subject motion. Neuroimage 59, 2142-2154 (2012).