To precisely evaluate the effect of the correction, 15-20 seeds were precisely and manually constructed on each of the cases. All of the seeds were positioned inside of the ventricles as far as possible from the edge of ventricles and corresponding partial volume voxels.
Non-uniformity is defined as the StdDev of the average seed signal taken over all the seeds.
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| Seeds are in Red on one of the slices |
We tried to find set of parameters that worked well on every single image.
See the Table of results below for each case
1st column: Case name
2nd Column: original non-uniformity (StdDev over seeds)
3rd Column: processed non-uniformity (StdDev over seeds)
5th column relative improvement of the correction =NUbefore/NUafter-1
Interestingly the results for N-cases where 10 times weaker than the results on "Non-N cases", 6% and 61% improvement correspondingly.
N3 correction was attempted but NU only worsened with all the parameter combinations we tried.
Overall these images are quite challenging for Non-uniformity correction, due to the low resolution and the ventricles being only 3-4 voxels thick. Note the original .NII files contained the resolution of (1,1,1)mm but this does not seem to be the case,
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