"All of the concerns that we have about the medical use of X-rays really don't apply to these devices," said Thrall, of the American College of Radiology and chief of radiology at Massachusetts General Hospital. "The exposure is extremely low and the energy of the X-rays is also very, very low. When X-rays are used for medical imaging purposes, they have to be energetic enough to get through the human body. The X-rays used in the backscatter machines in airports have such low energy that they literally bounce off the skin. That is what backscatter implies."