alt Hacker NewsAnywhere that US TSA runs the AIT scanners, you can opt out of them*.
That is domestic US airports plus airports like Toronto and Dublin where you, for practical purposes, clear into the US on foreign soil and land in the US as a domestic flight.
* - I think this only doesn't apply if your boarding pass got tagged with the dreaded "SSSS" enhanced screening tag, but that's a fairly rare corner case for most passengers.
My understanding, which may be wrong. It's been a few years since I did this dance.
You can opt out of the millimeter wave radar.
Opting out means you go through a metal detector, a 20-second pat-down and perhaps a hand swab for explosives sniffer.
If you have SSSS on your boarding card, that means the pat-down, hand swab and digging through your carry-on luggage happen whether you opt out of the mmwave or not.