Hallo Lucia,
I agree with other respondents that this is a mere terminological
issue. The rationale for modulated values is clear.
However, an argument can be made to the effect that volume, in a
physical sense, is not the same as what you obtain with modulated GM
segments. The reason is that the MR signal is sensitive to factors
that have nothing to do with volume, such as magnetic
susceptibility-induced artefacts, tissutal iron content, etc. The
rationale for VBM is that, when you model differences between groups
or from a regressor, the same procedure is applied identically to all
data. So, whatever differences you find, they may viewed as
differences between the groups. However, it is more difficult to make
conclusive claims on where these differences come from on the basis of
the MR signal alone. For example, iron affects the gradient-echo
signal, and there are differences in iron content arising from age.
Best wishes,
Roberto Viviani
Quoting Marko Wilke <[log in to unmask]>:
> Hello Lucia,
>
> I would share your understanding and don't think that the reviewer is
> correct in stating that the term "volume" may only be used for manually
> segmented images. The difference between density and is introduced in
> the process of spatial normalization, and the volume changes occurring
> during this process are re-integrated into the data by modulation with
> the Jacobian.
>
> The sum of all voxel values, taking into account voxel sizes, is
> therefore the same in native space and normalized, modulated gray mater
> maps (with the exception of the special case of only correcting for the
> non-linear parts of spatial normalization, a specialty of Christian
> Gaser's vbm toolboxes).
>
> The terminology may be vbm-community specific, which may explain the
> reviewers opposition. I don't think it is wrong, though, so I think you
> should get away with clear definitions in the revised manuscript :)
>
> Cheers,
> Marko
>
> Luc ía Vaquero wrote:
>> Dear SPM and VBM experts,
>>
>> I'm addressing to you in order to try to solve an open debate we're
>> encountering during the review of a VBM study.
>>
>> My colleagues and I explained in the manuscript that, since we
>> modulated the images by their Jacobian determinants, we were
>> looking at differences in Grey Matter Volume or Amount of GM.
>> However, one of the reviewers stated that, even though one takes
>> into account the Jacobian determinants, one's still dealing with
>> corrected probabilities after warping that are called
>> concentrations or GM density. So, he/she strongly recommends us to
>> use GM Density "because the term Volume can only be used when one
>> does manual segmentation of T1 images but not with VBM".
>>
>> That wasn't our understanding, we thought that GM Density is the
>> term when one doesn't do the modulation by the Jacobian
>> determinants (leaving this option by default in SPM) and Volume is
>> the term when one applies such modulation. All the information from
>> older messages regarding this topic and from previous reports and
>> manuals (by Ashburner and other users of VBM) seems to confirm our
>> original concept, but we still have doubts and we should be pretty
>> sure if we're trying to keep our terminology despite what the
>> reviewer is asking us to do (change the term GM volume by GM
>> density)...
>>
>> Thank you so much in advance for your time and help!
>> Best wishes,
>>
>>
>> Lucía Vaquero
>>
>> ------
>> Ph.D Student
>> Cognition and Brain Plasticity Unit
>> http://www.brainvitge.org/
>> University of Barcelona
>> Barcelona (Spain)
>> Tel: +34 93 402 10 38
>>
>>
>
> --
> ____________________________________________________
> PD Dr. med. Marko Wilke
> Facharzt für Kinder- und Jugendmedizin
> Leiter, Experimentelle Pädiatrische Neurobildgebung
> Universitäts-Kinderklinik
> Abt. III (Neuropädiatrie)
>
> Marko Wilke, MD, PhD
> Pediatrician
> Head, Experimental Pediatric Neuroimaging
> University Children's Hospital
> Dept. III (Pediatric Neurology)
>
> Hoppe-Seyler-Str. 1
> D - 72076 Tübingen, Germany
> Tel. +49 7071 29-83416
> Fax +49 7071 29-5473
> [log in to unmask]
>
> http://www.medizin.uni-tuebingen.de/kinder/epn/
> ____________________________________________________
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