[Eeglablist] fMRIb relative trigger location

[Rami K. Niazy]

Dear Yuko

The purpose of this parameter is to insure that EVERY part of an 
artifact segment (slice/volume depending on your triggers) is completely 
included between the positions of two successive triggers.  If you 
scroll through your data with the events on, then you can define the 
start/end as follows:

Start:  When you start seeing the artifact (or slightly before in the 
artifact-free area).
End:  When the next artifact starts using the above def.

We could define the start of the artifact a little earlier/later and 
still be ok, as long as the whole artifact segment is included.  Then 
you calculate the trigger relative to where you define the start of the 
artifact.

In your particular case, it seems that you get the trigger before any 
artifact appear (8ms before the the RF).  If this is indeed the case (as 
should be verified by scrolling through the EEG data with the events 
on), then just use 0.  

Rely on the EEG and Event data and not the pulse sequence when 
determining the trigger position.   There could be some delay between 
when the sequence sends the trigger and when it is recorded in the EEG.

I hope this helps.

Rami
 

Yuko Yotsumoto wrote:

> 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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-- 

Rami K. Niazy, DPhil Candidate
University of Oxford, Centre for Functional MRI of the Brain (FMRIB)
John Radcliffe Hospital, Headington, Oxford OX3 9DU, United Kingdom
Tel:    +44 (0) 1865 222739  / Fax:  +44 (0) 1865 222717
e-mail: rami at fmrib.ox.ac.uk  / URL:  http://www.fmrib.ox.ac.uk/~rami