weighting by surface area just let's you calculate the overall mean. Maybe an easier way to do this is to draw the exact region you want on fsaverage and use mri_label2label to map it to each of your subjects.
cheers Bruce
On Thu, 23 Dec 2010, j janssen wrote:
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:
- mri_annotation2label, gives you the labels.
- mri_mergelabels, merge only the labels you are interested in.
- 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:
- 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?
- 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 fischl@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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