WASHINGTON - High-tech security scanners that might have prevented the Christmas Day attempt to blow up a jetliner have been installed in only a few airports around the world, in large part because of privacy concerns over the way the machines see through clothing.

The technology is in place at 19 U.S. airports, while European officials have generally limited it to test runs.

Umar Farouk Abdulmutallab, the Nigerian accused of trying to ignite explosives aboard a Northwest Airlines jet as it was coming in for a landing in Detroit, did not go through such a scan where his flight began, at Amsterdam's Schipol airport.

The full-body scanner "could have been helpful in this case, absolutely," said Evert van Zwol, head of the Dutch Pilots Association.

But the technology has raised significant concerns among privacy watchdogs because it can show the body's contours with embarrassing clarity. Those fears have slowed the introduction of the machines.

Jay Stanley, public education director for the American Civil Liberties Union's Technology and Liberty Program, said that the machines essentially perform "virtual strip searches that see through your clothing and reveal the size and shape of your body."

Abdulmutallab passed through a routine security check at the gate in Amsterdam before boarding, officials said. He is believed to have tucked into his trousers or underwear a small bag holding PETN explosive powder, and possibly a liquid detonator.

Because such items won't set off metal detectors, the U.S. Transportation Security Administration, part of the Department of Homeland Security, has begun installing two types of advanced scanning machines that provide a more detailed picture.

These machines, which cost six figures each, screen airline passengers without physical contact. They can reveal plastic or chemical explosives.

Such scanners "provide the best protection for the widest range of threats," said Joe Reiss, a spokesman for American Science & Engineering Inc. The company makes machines for prisons, military agencies and other customers but does not have a contract with TSA, which employs 40 "millimeter wave" machines, which use radio waves to produce a three-dimensional image based on energy reflected back from the body.