[Freesurfer] Longitudinal Gray/white Contrast Measure

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Dear FreeSurfer experts,

I am currently running a longitudinal analysis in FreeSurfer v5.3 to look at changes in volume and gray/white contrast. I ran all the processing steps and used this command below to look at results for volume:

long_mris_slopes --qdec ./qdec/qdec.table.dat --meas volume --hemi lh --do-label --do-rate --time years --qcache fsaverage --sd $SUBJECTS_DIR

We also would like to look at change in gray-white matter intensity ratio over time (between 2 timepoints) as well, but there is no separate measure defined for it. I was wondering what we can do to calculate this.

Thank you. William