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Hi Arman,
Thanks a million. I understand what's the difference between those three variables.
One more questions. I saw this description from FSL-VBM website: "you would not be able to see any difference, if all these structures were perfectly aligned across the subjects (that's why you use limited degrees-of-freedom for the non-linear registration)"
This argument is really confusing. I think it would be correct if we just examine the changes in the shape. However, if we focus on DBM or TBM, or the continuum of the relative concentration and TBM, the perfect alignment is necessary. I do not understand what is the implication of that sentence.
Best,
Liang2012/4/2 Arman Eshaghi <[log in to unmask]>Hi,If I have understood your question correctly, Jacobian determinant of deformation field (or warp field in FSL manual) will produce modulated gray matter images. If you mean whether there would be any difference in the results of VBM, DBM and TBM, as VBM investigates gray matter concentration changes, DBM investigates the positions of structures within the brain, and TBM studies local shapes of the brain there could potentially be different results. Given the circumstances and your research question you will use different approaches, and you may get different results.All the very best,ArmanReference:Morphometry, John Ashburner and Karl Friston
On Apr 2, 2012, at 11:56 AM, liang wang wrote:Hi Arman,
Thanks so much for your clear explanations.
One more following question. Is that possible that both the modulated gray matter volume and Jacobian determinants could produce different results. When will this happen. And does the modulation step could more likely eliminate the between-group difference, compared with usage of Jacobian determinant?
Thanks,
Liang2012/4/2 Arman Eshaghi <[log in to unmask]>Hi,
In both cases that you have in mind Jacobian determinants could be used although in different concepts, in "optimised VBM" when you spatially normalize (or non-linear registration in FSL terminology) an image to a template when there is contraction there would be underestimation of gray matter concentration and when there is expansion there would be an over estimation of gray matter concentration. In this case you using Jacobian determinants can compensate this difference between an image and a template. This would be called "modulation".
Deformation Based Morphometry (DBM) will give you deformation fields specifically in within subjects' brains. You can use a Jacobian determinant of deformation fields in order to do a tensor based morphometry (TBM). Deformation fields usually give you too much information that may confound your analysis that's why you would want to use Jacobian determinants (and TBM) in longitudinal studies.
Both cases you will use Jacobian determinant for warping an image.
All the very best,
Arman
On Apr 2, 2012, at 4:42 AM, liang wang wrote:
> Hi,
>
> I have such basic question about the measures selected for group-level comparison using FSL-VBM. From the FSL-VBM instructions, I am instructed to use the modulated gray matter images for statistical tests. We also know that Jacobian determinant provides the voxel-wise compression and extension, and that a number of studies also use the deformation-based morphometry by using Jacobian determinant. So I am wondering whether someone can clarify the difference between those two measures in examining the abnormalities of brain structure related to diseases, and give some opinion how to correctly think of both guys. Thanks.
>
> Best,
> Liang
> --
> Liang Wang, PhD
> Neuroscience of Attention and Perception Laboratory
> Princeton Neuroscience Institute
> Princeton University
> Princeton, NJ, 08540
>
>
--
Liang Wang, PhD
Neuroscience of Attention and Perception Laboratory
Princeton Neuroscience Institute
Princeton University
Princeton, NJ, 08540
--
Liang Wang, PhD
Neuroscience of Attention and Perception Laboratory
Princeton Neuroscience Institute
Princeton University
Princeton, NJ, 08540