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In this experiment, the second slice was acquired 259.81 milliseconds
and the last slice was acquired 1910.02 milliseconds after the start of each time step.
In other words, the last slice is closer in time to
the subsequent time step (assuming a repetition time of 2000 milliseconds)
than to the time step to which it belongs.
This may cause significant problems in the statistical
evaluation of the data. The last slices are particularly
effected by this problem. The order of slice acquisition is usually
from ventral to dorsal, i.e. the dorsal slices are most
strongly effected.
The program 'vslicetime' corrects for this problem by using cubic-spline-interpolation.
The time courses in each voxel of each slice are virtually
moved backwards in time as if all slices had been acquired
simultaneously at the start of the time step.
'vslicetime' should be called after movement correction
('vmovcorrection') and before any
other type of processing takes place.
The program is typically called as follows:
vslicetime -in AC1T.v -out sAC1T.v
-del 5
Note that the first few time steps of the time courses usually have
extremely high values due to saturation effects. This can be corrected for
by replacing the first time steps of the time course by mean values.
The correction should be done early in the processing sequence and
is therefore part of 'vslicetime'. The parameter '-del' determines the
number of the first few seconds that are replaced by a mean value.
In some cases, it can be useful to apply the program 'vslicetime' only
for correcting the first few time steps (parameter '-del') without
performing a slicetime correction. This is
particularly the case if the interscan intervals are very large (> 5 seconds).
In this case, the parameter '-correction' should be set to 'false'.
If the interscan intervals (repetition times) are not constant, then an ASCII text file must be supplied. Each line in this file must contain the scan time in seconds for each scan. The parameter '-scanfile' is used to specify this file.