Just to chime in, you might find it useful to read some of the papers from
the FBIRN project, including those from G. Glover's group on multi-site
differences in fMRI and analysis approaches to help ameliorate those
differences.
Note also that it is not trivial to truly match "acquisition parameters"
across different scanner vendors. For example, differences in the timing
order of the slices with interleaved acquisitions, the default phase
encoding polarity, and the handling of bandwidth/echo spacing. And
different vendors use different k-space filtering algorithms, which can
make it difficult (or even impossible) to match the inherent spatial
smoothness of the reconstructed images, even if you acquire at the same
TR, TE, and voxel resolution. As for differences in field strength, the
BOLD contrast is inherently field strength dependent, so "combining" BOLD
data across field strengths is particularly problematic.
cheers,
-MH
--
Michael Harms, Ph.D.
-----------------------------------------------------------
Conte Center for the Neuroscience of Mental Disorders
Washington University School of Medicine
Department of Psychiatry, Box 8134
660 South Euclid Ave. Tel: 314-747-6173
St. Louis, MO 63110 Email: [log in to unmask]
On 1/17/13 9:53 PM, "Lorena Jimenez-Castro" <[log in to unmask]> wrote:
>Hello Dr. Jenkinson,
>
>Thanks Chris Watson and Pablo Velasco for your response. I got a follow
>up question for you all.
>
>Dr, Jenkinson Was your response taking into account that our groups are
>going to be balance across scanners? Does your explanation applies for
>scanners with different magnetic field strength or even for different
>scanners from different manufactures but same tesla?
>
>Regarding the fMRI data, I thought, that if I the groups are balanced
>across the two scanners and we use the same acquisition parameters in
>both sites, We could model the difference of the scanners in the design
>matrix (with one extra EV) and thus We would be able to analyze the
>combined data from the two sites. On the other hand I thought that to
>combine the DTI data could cause more problems in the analysis.
>
>So, since in our two data sets the groups (patients and controls) are
>going to be balance across the two scanners. That is, the number of
>patients and controls from one site are going to be the same number from
>the other site. Beside, exactly same acquisition parameters are going
>to be use in each site. All the above for the fMRI and for the DTI data.
>
>I just want to make sure that even so, it is not possible to combined the
>data (neither DTI nor fMRI), because we are going to use different
>scanners even if the scanners had the same magnetic field strength. Am I
>understanding this rightt??
>
>Thank you very very much. I greatly appreciate your help!!
>
>Lorena
>
>-- Lorena Jimenez-Castro, MD
>Postdoctoral Fellow
>Research Imaging Institute
>University of Texas Health Science Center
>8403 Floyd Curl Drive
>San Antonio, TX 78229
>(210) 567-8215- office phone
>(210) 567-8103- fax
>
>
>
>
>
>
>________________________________________________
>Re: [FSL] combine data from two different scanners and with different
>tesla
>Thursday, January 17, 2013 6:38 PM
>From: "Mark Jenkinson" <[log in to unmask]>
>To:[log in to unmask]
>
>Dear Lorena,
>
>I agree with Chris Watson, who answered your original email.
>It is very, very difficult to mix different scanners and different field
>strengths and in general we would advise against it as there are likely
>to be many non-biological factors that could result in changes that you
>would detect.
>
>If you really want to combine things then I would do independent analyses
>for the two different datasets and then combine the results, either
>qualitatively or quantitatively, as having similar results on different
>scanners is quite compelling. It also avoids the minefield of comparing
>changes in the data that could be attributable to the scanners only, as
>all your comparisons, to generate the individual statistical results,
>would be done on within-scanner data.
>
>Sorry I don't have better news.
>
>All the best,
> Mark
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