Passengers selected for a full-body scan can decline, but if they do, they must submit to full-body pat-downs by a TSA officer. The technology was introduced a couple of years ago, but U.S. airports have been slow to install the machines, partly because of privacy concerns raised by some members of Congress and civil liberties groups. 

On the one hand, the folks watching the monitors are going to be extremely jaded almost immediately, and on the other hand, I'm good with passing out guns and frangible ammo at the boarding gate, too, if you prefer ...