Dear Agnes,
> regarding longitudinal , multi-center trials, using DTI and FSL:
> We are planning a longitudinal study on stroke patients , and want
> to acquire DTI at different time points during recovery/training.
> Different centers are involved, so obviously different scanners.
> How would one best correct for the different scanners, in the
> analysis? (FA, tracts); and is it possible to combine PHilips and
> Siemens data?
> (1.5 versus 3 T is not possible at all, I guess; so just within the
> 3 T and the 1.5 T data)
it would obviously be best if everything could be kept constant, but
that is clearly not possible for a study like this.
Therefore I would say that you can vary almost any parameter you want
(i.e. field-strength, manufacturer etc) as long as you model it in
your design and is aware of the consequences of doing so. By modeling
it I mean for each scanner including a regressor with 1 if the scan in
question was performed on that scanner and a 0 otherwise. The
consequences of doing this is that you may potentially lose
sensitivity (depending on what scans are performed on what scanners).
To make this concrete, lets say you have acquired all your "initial
time points" on one scanner, then you get a grant to buy a new scanner
and acquire all the subsequent scans (time point two) on the new
scanner. You will then end up with a design where you have identical
"time" and "scanner" regressors which means that you have no
possibility to disambiguate between the 2. The highest sensitivity
(best design) would be the one where "scanner" and "time" are
orthogonal, i.e. when there is a equal number of time-point 1 and 2 on
all scanners.
If you can arrange the scanning so that each subject has all his/her
scans on the same scanner (which would be good) you will not need to
model scanner since that will be implicit in your subject/block
regressors.
I wouldn't worry about modeling different variances for different
scanners (I would assume the variance was dominated by the inter-
subject variance).
> as for scanning parameter: 60 directions, plus 6 volumes without
> diffusion weighting as the standard; do we have to choose the exact
> same TR and TE on all scanners? are there parameters that are
> optimal for longitudinal measurements?
60+6 is a very rich dataset, possibly slight overkill for TBSS (though
I'll let someone with more experience comment on that one). There are
as far as I know no special considerations for longitudinal data.
Good luck Jesper
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