Hi Maria
In general, you ideally want to keep all these parameters constant across all subjects. In your case, I assume you mean a TR range starting at 5000? TR=500 ms is very low, you will have a big SNR hit due to incomplete T1 relaxation. Assuming this is a typo, I would worry less. If not I would keep only data with long enough TR that allow T1 relaxation (maybe >5000 if you focus only on WM at 3T)?
Regarding combining gated vs non-gated data, it is difficult to directly model the effects of cardiac pulsation into your DTI parameters, as these depend a lot on the individual heart rate, timing of the volume acquisition with respect to systole/diastole, etc. Again this is not ideal, but what you should at least do is make sure to match the groups, so that e.g. you have the same number of gated subjects in each group.
Hope this helps
Stam
On 29 Jul 2013, at 13:57, maria estanqueiro wrote:
> Dear all,
>
> I have a set of 150 DTI scans that have very different repetition times, ranging from 500 to 13300.
>
> In addition there are a few scans that were acquired in a gated fashion and others that are non-gated.
>
> TE, slice thickness, matrix X Y Z, flip angle and number of directions are the same for all scans.
>
> Can I make group comparisons with TBSS using these DTI scans despite of their differences in TR and gated/non-gated?
>
> In case it is correct to do such analyses, should I include TR and gated/non-gated as covariates in Glm contrasts or should I carefully select subjects for each group to make sure they are paired or equivalent in terms of TR and gated/non-gated?
>
> Many thanks!
>
>
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