I have a follow-up question on this. I am writing a tutorial for this, and I am wondering if the requirement is that we should have isotropic data for correct estimation. Here is why I am asking this:
If I paint a 10mm diameter circle on the MRhead and calculate the area I get 243mm2, which is incorrect. If I resample the MRhead to be isotropic via CropVolume and paint the same area with the same diameter in a new segmentation, I get 309mm2 which is within measurement error. You get approximately 243mm2, if you scale the correct measurement by the slice spacing (1.3mm) which is different from XY spacing (1mm) in this dataset.
I attached the screenshots of the both segments (red from isotropic MRhead, and yellow is the original data).
I also suggest replacing row, column, and slice wording with the usual axial, coronal, sagittal.