Phoenix's Sky Harbor International Airport has started phasing in new body scanning machines that reveal hidden items a metal detector may miss.
The "backscatter" machine, which uses low-level X-ray beams to create a two-dimensional image of the body, went online last week inside the airport's Terminal Four.
Privacy watchdogs have raised concerns about the image of the body's shape that the machine produces.
The Transportation Security Administration says travelers can choose to opt-out of the high-tech screening and instead pass through a metal detector and physical pat-down.
A TSA spokesman says many people prefer the backscatter machines because they are a faster way to pass through security checkpoints.