At 13:24 15/02/2005 +0100, Hiske van Duinen wrote:
>Hello everyone,
>
>I have a question concerning the timing of events when you are
>scanning with a sparse-sampling scan protocol.
>We used a sparse sampling protocol to be able to measure EMG
>between scanning 2 volumes (we scanned a volume in 3 seconds -
>followed by 2 seconds without scanning). During the 2 seconds
>between volumes subjects produce force as well.
>When we specify a model, we say that the interscan interval is 5
>secs. However, then we have a problem: we cannot "tell" SPM that
>the duration of a volume is only 3 secs. Thus, if we say that an event
>started at scan 0.6: that means that the event started after 3 secs,
>thus after the first volume was taken, how will SPM handle this?
>Does SPM assume that the total volume took 5 seconds, and that
>the event started halfway the volume? Or is there a way to tell SPM
>what exactly happened?
You don't need to tell SPM that there is a difference between the TR and
the TA in specifying your model. Though you do need to specify this
information during slice-timing correction.
In the GLM, SPM assumes that all data in each scan is acquired
instantaneously. To correct for timing differences between slices you can
(a) use slice-timing correction during preprocessing, or (b) use temporal
derivatives in the analysis. You need to take care in setting:
SPM.xBF.T % number of time bins per scan
SPM.xBF.T0 % indexed time bin (see slice timing)
to ensure that the time at which SPM assume each scan occurs (T0) is
sometime around the midpoint of your acquisition, and that this corresponds
to the slice that you aligned the data to during slice timing correction
(if you use this). You also need to ensure that the timing of events
(specified in scans) is measured relative to this position in the scan.
There are numerous postings about this issue in the list archives. I would
suggest that you search for terms like:
fMRI_T0
slice timing
event-related
Hope this is helpful,
matt
****************************************************
Dr Matt Davis
MRC Cognition and Brain Sciences Unit
15 Chaucer Road, Cambridge, CB2 2EF
email: [log in to unmask]
tel: 01223 273 637 (direct line)
tel: 01223 355 294 (reception)
fax: 01223 359 062
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