[Eeglablist] fMRIb relative trigger location

Yuko Yotsumoto yuko at nmr.mgh.harvard.edu
Wed Oct 12 12:22:14 PDT 2005


Hi all,

I have eeg data recorded in fMRI scanner.
I've been using eeglab fMRIb plug-in for the EPI artifact removal.

I have a question about the "relative trigger location (in the advanced 
options)".

Here is the descriptions found in the tutorial.
 > Relative trigger location: During acquisition of each FMRI slice, the
 > MRI machine sends a trigger. The time at which the trigger is sent
 > relative to the start of the slice acquisition may vary from one MRI
 > machine to another. In this interactive box, you can define the
 > position of the trigger relative to the start of the slice. A value of
 > 0 indicates the trigger is at the exact beginning of the slice
 > collection and 1 indicates at the end. Usually the trigger is much
 > closer to the beginning than the end. The default value is 0.03.

I am wondering what would be the definition of the beginning and the end 
of "the slice acquisition".

Does it start at "the peak of RF pulse (0TE)" or "the moment the scanner 
send out a RF pulse (about 1 ms before the RF peak)"?

Does it end at "the end of read out", "the end of crushers", or "defined 
by the TR/#slices"???

Our system send out a trigger pulse at 8 msec before the RF peak, and we 
are collecting 35 slices with TR=4sec. Readout duration is 64x(.44ms), 
so the time from the RF peak to the end of readout is 44.08 msec. I'd 
like to figure out the accurate "relative trigger location" in our sequence.

Thank you!
Yuko Yotsumoto



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