On 09/12/2012 08:43 AM, Bruce Fischl wrote:
> Hi Heath
>
> yes, I think it's a bad idea for thickness just as it would be for
> VBM. ADNI is a bit of a special case as they went to great lengths to
> make their acquisitions uniform across scanner platform, but I would
> still look to see if there is a significant scanner effect in
> analyzing that data.
>
> cheers
> Bruce
>
>
>
> On Tue, 11 Sep 2012, Heath Pardoe wrote:
>
>> Hi all,
>> I was wondering if there was a consensus view on the optimal way to
>> approach morphometric analysis (cortical thickness,
>> volumetry & associated analyses) of MRI data acquired on multiple
>> scanners.
>>
>> From VBM literature & our own experience it appears that comparing
>> MRI scans of patients from one scanner with controls scanned
>> on another is a bad idea, because there are systematic differences
>> that will manifest as very significant GM
>> volume/concentration/density differences following statistical
>> analysis when these differences are unlikely to really exist ie.
>> we get a lot of false positives.
>>
>> Definitive findings on similar analyses using cortical thickness
>> mapping seem harder to come by. There's the Han study from 2006
>> and Dickerson 2008 but both of these seem to be empirical analyses of
>> mean absolute error or related measures rather than
>> pitting scanner 1 subjects against scanner 2 subjects in a
>> statistical analysis (please correct me if I haven't looked hard
>> enough). Wonderlick et al 2009 is closer in terms of statistical
>> analysis but these were done on one scanner so don't really
>> address the multicenter issue.
>>
>> To my mind it seems likely that cortical thickness analysis would
>> have the same problem as VBM and would require controls to be
>> scanned on the same scanners as patients to account for inter-scanner
>> variability. Does anyone have any opinions on this? Or,
>> even better, any data so we can tell how much of a problem this is?
>>
>> In a related question, the ADNI study uses a phantom to correct an
>> image for gradient nonlinearities in different scanners, and
>> then pools this corrected data from multiple scanners into each
>> control or patient group, and subsequent morphometric analyses
>> are done without worrying about the multi-site issue. Does anyone
>> have an opinion on the validity of this approach?
>>
>> I guess one way to investigate it would be to look at vertex-wise
>> across-subject variance in coregistered cortical thickness
>> maps both with and without the phantom-based correction procedure; if
>> the variance was lower in the phantom-corrected variance
>> map this would provide good evidence that it's worth the effort of
>> undertaking this procedure. I would be keen to do this
>> analysis but unless I'm mistaken the ADNI website only provides
>> images that have already been processed to correct for gradient
>> nonlinearities.
>>
>> An alternative approach that has been proposed is to scan a group of
>> the same controls on multiple scanners to characterise
>> between-scanner differences. The practicality of this approach breaks
>> down as the number of centers increases. Again I would be
>> interested to hear if anyone has experience in using this approach
>> compared to others.
>>
>> In summary, there seem to be three ways to overcome inter-scanner
>> variance:
>> 1. scan different controls at each site
>> 2. scan a phantom at each site (and probably scan controls too anyway)
>> 3. scan the same group of controls at all sites
>>
>> I would be interested in hearing which approach people think is best,
>> and why.
>>
>> Thanks if you're still reading!
>> Best wishes
>> Heath
>>
>>
>
>