Kochunov and Bruce,
Thank you very much for your answers.
Unfortunately we don't have the kind of data required to perform the before/after comparison, but I'll keep in mind the procedure for the future.
Greetings,
Nancy

-----Original Message-----
From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf Of kochunov
Sent: Thursday, April 21, 2011 1:00 PM
To: [log in to unmask]
Subject: Re: [FSL] follow up Multi-scanner study

Depends on what was done as the part of the upgrade.
Typically, you'd want to image several local people multiple times before
and after the upgrade. Then if there is no systemic differences between two
groups you can declare the data equvivalent.
pk


-----Original Message-----
From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf Of Bruce Fischl
Sent: Thursday, April 21, 2011 3:16 PM
To: [log in to unmask]
Subject: Re: [FSL] follow up Multi-scanner study

Hi Nancy,

it depends on what happened during the upgrade. The best thing to in
these situations is to scan some healthy controls immediately before and
after the upgrade to evaluate changes (and phantoms of course).

There are many, many things that can happen during an upgrade that will
mess up any longitudinal analysis.

cheers,
Bruce

-----Original Message-----
From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf
Of Nancy Estevez
Sent: Thursday, April 21, 2011 1:18 PM
To: [log in to unmask]
Subject: [FSL] follow up Multi-scanner study

Hi FSL community
Following up this kind of analysis...
I have two groups of controls and patients, all acquired in the same
scanner, but in two different moments, and between the first group and the
second an upgrade on the scanner's protocols was performed. Is there any
hope I could combine these groups?
Thanks in advance for your attention...
Nancy

-----Original Message-----
From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf
Of Michael Harms
Sent: Thursday, April 21, 2011 9:31 AM
To: [log in to unmask]
Subject: Re: [FSL] Multi-scanner study

Hello Jon,

In principle, you can only ignore the possible effect of scanner if each
group was balanced across the three scanners, and even then, accounting
for scanner might soak up some residual variance and thus improve your
power to detect group differences.

However, it sounds like the patient groups were not at all balanced
across scanners, but in fact each patient group was acquired on a
completely separate scanner.  In this case, the effect of patient group
and scanner is hopelessly confounded, and cannot be disentangled.
Sorry, but if that is indeed the case, there is nothing you can do, and
I would say that the data is not suitable for group comparisons.

Best,
-MH

On Thu, 2011-04-21 at 09:49 +0100, Jon Brock wrote:
> Hi,
> Any advice appreciated.
>
> I have data from 4 groups of subjects (Controls and 3 Patient groups)
> acquired on 3 different scanners. If possible I would like to combine
> these data together but I am unsure how to model scanner as a covariate.
If I
> combine the Control data together then all three scanners will be
represented
> in this group. Would it be correct to assume that I can safely compare any
> patient group with the control group and not worry about any potential
effect of
> scanner?   What happens if I want to  compare between patient groups? In
this case
> each Patient group has been acquired on a different scanner so I think I
need to
> make sure there are no main effects of scanner or scanner x group
interactions? Any advice
> on what a suitable design matrix and contrasts would look like would be
greatly appreciated.
>
>
> Cheers

CLAIB 2011                          www.claibhabana2011.sld.cu
V Congreso Latinoamericano de Ingenier?a Biom?dica/ V Latin American
Congress on Biomedical Engineering/ V Congresso Latino-Americano de
Engenharia Biom?dica
Organizado por/ Organized by/ Organizam:
Consejo Regional de Ingenier?a Biom?dica para Am?rica Latina (CORAL)
Sociedad Cubana de Bioingenier?a
Mayo/May/Maio 16 - 21, 2011, Palacio de las Convenciones, La Habana, Cuba

CLAIB 2011                          www.claibhabana2011.sld.cu
V Congreso Latinoamericano de Ingenier?a Biom?dica/ V Latin American Congress on Biomedical Engineering/ V Congresso Latino-Americano de Engenharia Biom?dica
Organizado por/ Organized by/ Organizam:
Consejo Regional de Ingenier?a Biom?dica para Am?rica Latina (CORAL)
Sociedad Cubana de Bioingenier?a
Mayo/May/Maio 16 - 21, 2011, Palacio de las Convenciones, La Habana, Cuba