That woman, who had a letter from the TSA saying that breast milk didn't need to go through the xray scanner, didn't have a fun time when she asked for her breast milk to not be xrayed.

http://rt.com/usa/154672-tsa-breast-milk-settlement/

But she did get a settlement and so -if you have the time and know your rigts and can keep calm[1]- it might be a good idea to opt out.

[1] calm because otherwise tasers or guns.