Dear Mahinda,
I'm afraid that this is really quite problematic and very difficult or
impossible to adequately correct for. If it is extremely valuable
data then it might still be worth processing and simply displaying
the results found for both the group of interest and the control
group, in order to show the reader how much change might have
been induced by the change in scanner and where it appears.
This then let's the reader decide for themselves how trustworthy
the results are. However, if the structures of interest are quite
different in the group of interest (e.g. due to major atrophy) then
this approach is also not that good as the location and amount of
change may be quite different to the control group.
I'm sorry I do not have better news, but in longitudinal studies
it has been shown that it is very important to try and control
the scanning environment as much as possible, and the change
you are talking about is massive and hence problematic.
All the best,
Mark
On 15 Jul 2010, at 14:49, Mahinda Y wrote:
> Dear Experts,
>
> We are carrying out an analysis of structural data recently acquired
> on a 3T GE Excite II scanner (coronally acquired 1.1mm thick slices)
> using FIRST. However these same subjects have also been previously
> scanned (8-9 years ago) on our old 1.5T GE Signa scanner (coronally
> acquired 1.5mm thick slices). We also have a group of control
> subjects who have been scanned on both the 1.5T and 3T scanners
> (scanned no more than a couple of months apart).
>
> Ideally we would like to compare measurements at the 2 timepoints in
> patients, and see if there has been any change in subcortical
> volumes - is this possible, given the difference in scanners, and if
> so what would be the best approach. Is it possible to analyse the
> control dataset from the 1.5T and 3T scanner using FIRST and derive
> some sort of "scanner specific correction factor" that can then be
> applied to the analysis of patient data ?
>
> Thanks.
>
> Mahinda
>
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