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

The body-scanning technology is in at least 19 US 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 Schiphol airport.

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

The technology has raised significant protests 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 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 will not set off metal detectors, the US 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, each of which costs in the hundreds of thousands of dollars, screen airline passengers without physical contact. They can reveal plastic or chemical explosives and nonmetallic weapons.

Such scanners "provide the best protection for the widest range of threats", said Joe Reiss, vice-president of marketing for American Science & Engineering Inc. The company makes machines for prisons, military agencies, foreign customs patrols and other customers but does not have a contract with TSA.

TSA has deployed 40 "millimetre wave" machines, which use radio waves to produce a three-dimensional image based on energy reflected back from the body.

Six of those machines, which are made by L-3 Communications Holdings Inc., are being used for what TSA calls "primary screenings" at six US airports: Albuquerque, New Mexico; Las Vegas, Nevada; Miami, Florida; San Francisco, California; Salt Lake City, Utah; and Tulsa, Oklahoma.

This means passengers go through the scans instead of metal detectors, although they can elect to receive a pat-down search from a security officer instead.

The remainder of the machines are being used at 13 U.S. airports for secondary screening of passengers who set off a metal detector: Atlanta; Baltimore/Washington; Denver; Dallas/Fort Worth; Indianapolis; Jacksonville and Tampa, Florida; Los Angeles; Phoenix; Raleigh-Durham, North Carolina; Richmond, Virginia; Ronald Reagan Washington National; and Detroit. Travelers can opt for a pat-down instead in those instances as well.

The agency also says it has bought 150 "backscatter" machines, which use low-level X-rays to create a two-dimensional image of the body, from Rapiscan Systems, a unit of OSI Systems Inc.

Those machines, which cost $190,000 each, are expected to be deployed in US airports in 2010.

"The machine gives a very accurate and very precise image of things on the body that are not the body," said Peter Kant, executive vice-president of global government affairs for Rapiscan.

Last June, however, because of privacy concerns, the House voted 310-118 to prohibit the use of whole-body imaging for primary screening.

The measure, still pending in the Senate, would limit the use of the devices to secondary screening.