why devisions by 14 here self.phantom_ski_region.major_axis_length / 14 in QC3 implementation? any idea

As stated in the function documentation right above that line:

The radius of the phantom in pixels; the value itself doesn’t matter, it’s just used for relative distances to ROIs.

There is no rhyme or reason to the use of 14 other than for ROI distances from the center.

thank you for your explanation Cordially

I would like to know why we calculate the MTF just at 50% and 80% please, what is the meaning of this two value (MTF just at 50% and 80%) QC-3 results:
MTF 80% (lp/mm): 2.02
MTF 50% (lp/mm): 3.28 Thanks

MTF 50% is a common metric for image quality. 80% was also used just for convenience. The entire plot is given in the PDF for your convenience so you can see the rMTF at all values.