We know that the TSA doesn’t actually catch contraband through the checkpoint the vast majority of the time. So their procedures aren’t actually meaningful for security, leaving aside that they’ve never caught a terrorist.
But now with news that the TSA will make the imaging devices mandatory for some passengers, it’s likely that use of these machines violate the fourth amendment since a Federal Appeals Court ruled in EPIC v. DHS that the scanners were constitutionally permissible because they’re optional.