The body scanners—which use backscatter X-ray and millimeter wave technology—most likely wouldn’t detect an implanted explosive because they’re designed to find objects on the body—not inside it.

Steven W. Smith, who invented the X-ray body scanner used at checkpoints, has said that the images do penetrate slightly beneath the skin, allowing the operator to see shin bones. But they can’t see ribs or internal organs.

And while airport metal detectors frequently alarm on hip and knee implants, they would be incapable of detecting a device made of plastic.

None of the machines the TSA uses can see inside body cavities or underneath skin like a medical X-ray can. Several critics, including members of Congress, have cited this inability to challenge TSA’s plans to deploy the scanners at nearly every security lane by 2014. As an example, they point to the 2009 attack at a Saudi palace by a suicide bomber who detonated explosives secreted in his rectum.