Dear all,
I am a bit confused. I do not understand why the mean_ph, mean_th files of the bedpostx output are 3d files?
I thought they are describing the spherical coordinates (angles) of the principle diffusion directions thus I supposed to see something like a scalar value (degree, rad) for each voxel. So for example in voxel one phi has a scalar value of x1 and in the next one x2 ...
I also checked the histogram of the mean_phi files but in this case again I do not understand the files. Some of the values are very high, some are almost equal to zero.
For my work I want to compare the principle diffusion directions per voxel calculated from different measurements via bedpostx command.
Therefore I need to calculate some statistic values (mean, covariance matrix, trace) to check which measurement is the best to find direction information.
My idea was it to calculate of out of all measurements the mean (of all mean_phi,mean_theta) and describe it as my true value (reference). Then compare each single measurement with this true value via calculating the variances (trace of the covariance matrix).
1. So how can I interpret the mean_phi? Why is it 3d and for what exactly are the values stand for?
2. Is my idea for the statistical analysis correct? If not, how can I do it correct. If yes, is there still a better way to do it? (I have read the https://fsl.fmrib.ox.ac.uk/fsl/fslwiki/FDT/UserGuide?action=AttachFile&do=view&target=dyads.jpg but I do not understand the equations, cause there are two different indices (i) but no explanation for it. Also I do not understand why the dispersion is equal to the (1-largest eigenvalue)) .
3. Do somebody know where I can find some more detailed explanation of the dispersion calculation?
thank you for any kind of help
cheers
Max
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