Dear Tom,
I have to confess we have been using the same constants
in the output nonlinearity for both our 1.5 and 3.0T data.
The reason is that there are sufficient conditional dependencies
among the [uninteresting] biophysical parameters for each region
to fit the data and these parameters are largely (conditionally)
independent of the [interesting] coupling parameters.
Put simply, as long as the hemodynamic part of the DCM is flexible
enough to model each regional HRF accurately, the conditional
density of the coupling parameters should be fairly stable.
The current priors provide a quite flexible model. Note that the
hemodynamic part does not have to be biophysically realistic (although
the DCM model is reasonably realistic).
Having said this, we have never checked the impact of changing
the output constants and field strength. This would be an interesting
exercise that may motivate making these constants free parameters
with relatively tight priors. I would only do this if you were interested
in this issue (and wanted to write a short technical note on it).
I hope this helps - Karl
PS: I have a vague memory that someone has already looked at this.
Perhaps they may respond to this enquiry?
At 13:59 15/07/2005 +0200, you wrote:
>Dear Dr. Friston,
>
>I would like to conduct my next DCM study on a 3T system. What worries
>me is that the coefficients in the Balloon model are valid for 1.5 T and
>a TE of 40 ms. In your methodological paper on DCM you show that
>violations of priors on hemodynamic or biophysical parameters (in a
>certain range)does not greatly affect normalized coupling parameters. Do
>you think that DCM is robust changes in T2* and Intra/Extravascular
>contributions in gradient echo BOLD induced by moving from 1.5T to 3T or
>would it be necessary to evaluate these changes first and modify the
>coefficients accordingly?
>
>Sincerely yours
>
>Tom Ethofer
>
>
>
|