Hi Tina,

> (e.g. to compute a general cortical thickness average for
> the inferior frontal gyrus using the pars orbitalis, pars opercularis and
> pars triangularis data).

instead of "merging" existing thickness-data you could create a new
label that constitutes your regions of interest. something like:

1. mri_annotation2label, gives you the labels.
2. mri_mergelabels, merge only the labels you are interested in.
3. mris_anatomical_stats, use the newly created label (e.g. inferior
frontal gyrus) to compute thickness, surface area, volume etc.

>Average cortical thickness= [(part 1 cortical thickness*part 1 surface area) + 
>(part 2 cortical thickness*part 2 surface area) + …]/ >(part 1 surface area + 
>part 2 surface area + …)

my 2 cents:
1. if you have an a priori hypothesis dealing with the mean thickness
of certain region (e.g. inferior frontal gyrus, IFG) then your should
calculate the mean thickness of IFG. if you have an a priori
hypothesis about the mean thickness of a certain part of the IFG then
you should calculate the mean thickness for that part. i think your
approach doesn't follow this, you are going from subregions to IFG
thereby weighing for surface area of the subregions, what's your a
priori reason for doing this? why not measure the pars-regions
directly and forget about IFG?

2. the linear relationship between thickness and surface area. e.g. if
mean thickness is equal in all subregions that make up the IFG, you
give more weight to the region with a larger surface area, why?

best,
-joost




On Thu, Dec 23, 2010 at 4:00 AM, Bruce Fischl
<fis...@nmr.mgh.harvard.edu> wrote:
> Hi Tina,
>
> I think weighting by surface area is a reasonable approach.
>
> cheers
> Bruce
> On Wed, 22 Dec 2010, tina roostaei wrote:
>
>> Dear all,
>>
>> I have access to a dataset of freesurfer segmentation  products (i.e.
>> cortical
>> volume, surface area, cortical thickness average  and standard deviation)
>> for a
>> number of patients. I wanted to merge the  data of some segmented parts
>> (e.g. to
>> compute a general cortical  thickness average for the inferior frontal
>> gyrus
>> using the pars orbitalis, pars opercularis and pars triangularis data).
>> I wanted to know if it is right to compute the average cortical thickness
>> using
>> the following formula:
>> Average cortical thickness= [(part 1 cortical thickness*part 1 surface
>> area) +
>> (part 2 cortical thickness*part 2 surface area) + ÿÿ]/ (part 1 surface
>> area +
>> part 2 surface area + ÿÿ)
>> BTW,  I also checked whether the cortical volume equals ÿÿsurface
>> area*average
>> cortical thicknessÿÿ, to see if I could use the following formula
>>  instead:
>> Average cortical thickness= (part 1 cortical volume + part 2  cortical
>> volume +
>> ÿÿ)/ (part 1 surface area + part 2 surface area + ÿÿ)
>> But it simply proved wrong.
>>
>> Any suggestion is greatly appreciated.
>>
>> Thanks in advance.
>> Tina
>>
>>
>
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