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Hi Steve and Janssen Thanks for your comments, I understand your points, in general with one value, e.g., '1' for the images with a particular voxel size and '2' for the other it is possible to remove the effect due to different sampling schemes. However, in my particular case the data is not well matched across the sampling schemes. That is, one group was measured with the first sampling scheme and the other with the second one, so, in this case the linear model is not able to separate the effect due to different sampling schemes from the effect due to different groups. The fit line will pass exactly through the center of the points that correspond to each group, consequently the distance between the line and the center of the points of both groups will be equal to zero, reason why the null hypothesis (i.e., not difference between the groups) will be fulfilled. For this reason I was interested in accurate interpolation methods, however, as Steve has pointed, the interpolation gives rise to bias that can not be modellable in a direct way. Any suggestion will be very useful for me, Thanks again and Best Regards, Erick J. Canales-Rodríguez (+34) 93 652 99 99 ext. 255 Unitat de Recerca, Benito Menni CASM (http://www.hospitalbenitomenni.org/Default.aspx) Sant Boi del Llobregat (Barcelona), Espanya/España/Spain Benito Menni Complex assistencial en Salut Mental ________________________________________ De: FSL - FMRIB's Software Library [[log in to unmask]] En nombre de Steve Smith [[log in to unmask]] Enviado el: martes, 23 de diciembre de 2008 18:51 Para: [log in to unmask] Asunto: Re: [FSL] TBSS with different diffusion data? Hi I would just try (e.g.) '1' for one voxel size and '2' for the other, and then demean that EV. Cheers -------------------- Stephen M. Smith, Professor of Biomedical Engineering Associate Director, Oxford University FMRIB Centre FMRIB, JR Hospital, Headington, Oxford. OX3 9 DU, UK +44 (0) 1865 222726 (fax 222717) [log in to unmask] http://www.fmrib.ox.ac.uk/~steve ---------------------- On 23 Dec 2008, at 13:41, "Erick Canales Rodriguez (Benito Menni CASM)" <[log in to unmask]> wrote: > Hi Steve, > > Thanks for your reply, > In your opinion the spatial resolution of the data can be considered > as a covariable in the general linear model? > For example, one additional column (EV) to include the volume of the > voxel. > > Best regards, > > Erick J. Canales-Rodríguez > > (+34) 93 652 99 99 ext. 255 > > Unitat de Recerca, Benito Menni CASM (http://www.hospitalbenitomenni.org/Default.aspx > ) > Sant Boi del Llobregat (Barcelona), Espanya/España/Spain > > Benito Menni Complex assistencial en Salut Mental > ________________________________________ > De: FSL - FMRIB's Software Library [[log in to unmask]] En nombre de > Steve Smith [[log in to unmask]] > Enviado el: martes, 23 de diciembre de 2008 14:02 > Para: [log in to unmask] > Asunto: Re: [FSL] TBSS with different diffusion data? > > Hi, > > See previous posts on this: in general people are very wary of trying > to combine data across different protocols. You will have a bias in > pretty much all diffusion-derived measures, which may not be trivally > modellable, and is almost certainly not solvable by re-interpolating > your data, sadly. However, IF you put in the protocol labelling as a > confound regressor to randomise, and IF your regressors of interest do > not correlate strongly with this, and IF your data is well matched > across the protocol groups, then you might get away with this........ > > Cheers. > > > > On 23 Dec 2008, at 10:33, Erick Canales Rodriguez (Benito Menni CASM) > wrote: > >> Hello FSL, >> >> My interest is to compare two groups of subjects with TBSS, however, >> the diffusion MRI data corresponding to each group was recorded with >> a different spatial resolution (in the same MRI machine). That is, >> the same b-value and diffusion gradient directions but different >> voxel size, the first group: 1.13x1.13x5 mm and the second one: >> 0.94x0.94x5 mm. >> My question is, it is possible to use TBSS to infer FA differences >> in groups with data at different spatial resolutions? Typically this >> procedure is not a good practice, however, I consider this >> possibility due to the data was measured with a resolution lower >> than the resolution of the template used in TBSS: 1x1x1 mm. >> A possible solution may be to interpolate the resulting FA images to >> the same spatial resolution prior to the TBSS analysis. A second >> variant may be to perform the interpolation directly on the measured >> diffusion data. There is any script in FSL to perform accurately >> these interpolations? >> Many thanks in advance, >> >> Erick J. Canales-Rodríguez >> >> (+34) 93 652 99 99 ext. 255 >> >> Unitat de Recerca, Benito Menni CASM (http://www.hospitalbenitomenni.org/Default.aspx >> ) >> Sant Boi del Llobregat (Barcelona), Espanya/España/Spain >> >> Benito Menni Complex assistencial en Salut Mental >> ________________________________________ Este mensaje se dirige exclusivamente a su destinatario y puede contener información privilegiada o confidencial. 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