Since you have a two-observations per subject, you are trying to evaluate the change  in VBM over time.

Your flexible factorial design should have 2 factors (time and subject). The design matrix that results should have a column for time 1, a column for time 2, and a column for each subject. If the design matrix does not have this setup, then you need to set it up again. Make sure that you have a main effect for time and main effect for subject.

I am saying that you shouldn't add age or TIV to the flexible factorial model. If you want to look at TIV or age and change over time, you could subtract time 1 from time 2 for each subject and then use a one-sample t-test on these differences and add TIV or age as covariates. In this way you are saying the the change in VBM might be effected by TIV or age. 

The accuracy of TIV using T1-weighted images isn't perfect, which could explain some of the variation over time.

Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Office: (773) 406-2464
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On Thu, Jan 19, 2012 at 11:33 AM, Kairys, Anson <[log in to unmask]> wrote:

So, just to clarify, are you suggesting I don’t perform flex factorial? What do you mean by 2+N columns? Sorry I am a little confused….I do understand that TIV does not increase across time, however I did find local decreases in just 14 days after drug trx. But you are saying I should not add TIV as a regressor?

 

Should I not be doing a flex factorial, and instead a 2-sample ttest?

 

Thanks for your help

 

Anson Kairys

From: MCLAREN, Donald [mailto:[log in to unmask]]
Sent: Thursday, January 19, 2012 11:25 AM
To: Kairys, Anson
Cc: [log in to unmask]
Subject: Re: [SPM] ADDING TIV TO DENSITY ANALYSIS - VBM

 

Repeated-measure ANCOVAs should not be performed as the statistical field, last I checked, does not have a uniform way of doing the analysis. Additionally, you don't account for any more variance as the subject columns are collinear with TIV and age. Remember TIV does not change with time, which is why its often used as a covariate in between-subject analyses.

 

Your flexible factor model should have 2+N columns where N is the number of subjects.


Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Office: (773) 406-2464
=====================
This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED
HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is
intended only for the use of the individual or entity named above. If the
reader of the e-mail is not the intended recipient or the employee or agent
responsible for delivering it to the intended recipient, you are hereby
notified that you are in possession of confidential and privileged
information. Any unauthorized use, disclosure, copying or the taking of any
action in reliance on the contents of this information is strictly
prohibited and may be unlawful. If you have received this e-mail
unintentionally, please immediately notify the sender via telephone at (773)
406-2464 or email.


On Thu, Jan 19, 2012 at 10:26 AM, Anson Kairys <[log in to unmask]> wrote:

Hi All,

I am doing a VBM analysis pre and post drug treatment. I found my results were improved when I added total intracranial volume as a regressor, along with age. This is using un-modulated images (density).

Would you find this to be acceptable when conducting a flexible factorial analysis?

thanks

AK

 

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