Dear Frederique,
I'm not sure how much you can infer from a sample size of only 4 patients /
4 controls, but in the same model you can try:
(postT patients - preT patients) vs. (postT controls - preT controls)
This should control for change in scanner as well as change in time.
Doug Burman
At 06:50 PM 7/18/2006 +0100, Frederique Liegeois wrote:
>Dear SPM users,
>
>I have an unusual question and hope that one of you can help me. I have
>conducted a preliminary study investigating the brain changes associated
>with a course of treatment in 4 patients. The patients were therefore
>scanned before therapy (preT) and after therapy (postT) in a simple Task
>vs Rest fMRI block design. To control for the effect of "time elapsed
>between preT and postT", I have also scanned matched controls twice at
>about the same time, but the controls did not undergo any treatment. In
>theory this enables me to explore the brain regions that are under- and
>overactive in the patients relative to the controls before and after
>therapy, as well as the difference.
>
>HOWEVER, my problem is that, due to unforeseen circumstances, the
>treatemnt did not start at the time originally planned, and our scanner
>was REPLACED in the middle of the study.
>This means that my preT data were acquired on Scanner#1 and my postT data
>on Scanner#2. The TRs and TEs are different.
>
>Had this not happened, I would have put all the data in a fixed effect
>model and tested the interaction:
>(preT Patients vs. PreT controls) vs (postT Patients vs. postT controls)
>1 -1 -1 1 and -1 1 1 -1
>Unfortunately I cannot do that because the scanning parameters are different.
>
>I am wondering whether there is any way I can still compare my preT and
>post T data within SPM2, using a method that would be acceptable to a
>reviewer.
>
>For instance, would it be acceptable/legal to bring my Patients vs
>Controls contrasts (con1_preT and con2_postT) to a second level analysis
>to explore the interaction I am intested in?
>If so, is that considered a one-sample t-test, a two-sample t-test, or a
>paired t-test? These are the same subjects (so repeated measures) but as
>they were not scanned on the same machine I am not sure about which one to
>use to be as conservative as possible.
>
>Should I instead/additionally compare (Patient preT vs Patient postT) and
>(controls preT vs control postT) in a second-level analysis (two-sample t
>test), and bring those two contrasts to a third-level?
>
>Is there another way I can process my data that would enable me control
>for the effect of scanner change?
>
>Any help would be greatly appreciated as these treatment studies are very
>time-consuming and I really hope I can still use my data.
>
>Many thanks in advance for your help,
>
>Frederique
>
>--
>Frederique Liegeois, PhD
>UCL Institute of Child Health
Dept. of Communication Sciences & Disorders
Northwestern University
2240 Campus Drive
Frances Searle Building, Room 2-356
Evanston, IL 60208
phone 847-467-1549
fax 847-491-4975
email: [log in to unmask]
|