In one of the stranger technological twists of the 21st century, X-ray vision has gone from male teenage fantasy to airport screening reality.

The devices have been nicknamed "naked scanners" for a reason: when you walk through them, as many travelers will this Christmas weekend, someone in an undisclosed location will see through your clothes and check your body for weapons or explosives. At the same time, a privacy/modesty filter masks your face and genitalia to some extent - but not your flab - and the image is discarded, the feds say.

The phone-booth-size scanners sit beside the metal detectors. Passengers can opt out, as some have encouraged, but then they have to get a thorough pat-down.

Some scientists are raising concerns that these devices are too easy for would-be terrorists to foil. A few are also questioning the safety of some of the devices, since about half expose passengers to X-rays - though the dose is tiny compared with a typical medical X-ray.

So far, 464 scanners have been installed in 75 airports; about half use low-dose X-rays. The others - including the one installed in October in Philadelphia International Airport's Terminal F - create images with radio waves and pose no known health risk.

The scanners were originally seen as a secondary measure, experts said. But they were rushed into routine security after the so-called underwear bomber incident last Christmas, when a man on a flight from Amsterdam to Detroit tried to ignite a small amount of the plastic explosive PETN sewn into his underwear.

"It's amazing that this whole [scanner] thing has happened totally outside the normal scientific community, without peer review and discussion," said physicist Peter Rez of Arizona State University.

Whom can we credit (or blame) for inventing these scanners? The X-ray scanners were invented in 1991 by Steven Smith, a former police officer with advanced degrees in physics and engineering. Smith, now president of the engineering company Tek84, said there wasn't much of a market at first.

Eventually, about 300 were used in prisons and military environments. Smith sold his scanner design to another firm before it was adopted by the Transportation Security Administration, but he said Tek84 was working on an updated model that he hopes TSA will adopt.

Meanwhile, scientists at Pacific Northwest National Lab were developing a way to penetrate people's clothing with high-frequency radio waves.

These so-called millimeter-wave scanners are sold through a private partner, L3. Before L3 brought the scanners to airports, Horsham-based Intellifit introduced them to shopping malls, where they're supposed to help shoppers find the perfect pants.

How do they work? In the millimeter-wave machine, two antennas rotate around passengers and transmit the waves, which have a frequency between microwaves and infrared. The waves go through clothing but are stopped and reflected by water, the main component of the human body.

The relative degree of reflection or transmission will depend on the material these waves strike, so the scanners should pick up items hidden under clothing as darker or lighter regions, he said. That can signal where on a passenger's body to search.

The X-ray scanners make images using what's called backscatter. When used in medicine to image a fracture, the picture is created by X-rays that travel through the body, Smith said. With backscatter, the image comes from X-rays that bounce off the body.

Different materials will scatter X-rays to different extents, Smith said, so the images will reveal objects inside a person's clothing.

The X-ray scanners can create a detailed image of a body, and are used that way in European airports, said Amit Verma, Tek84's director of government affairs. In the United States, the machines "superimpose a filter so it's less revealing," he said, though used that way, it's also slightly less effective.

TSA spokesman Ann Davis said there was no danger of a breach of privacy in any case. The workers who get to view the images are in a remote, private room and do not save the images, she said.

Will they really catch potential terrorists or is this just more "security theater"? The only thing experts agree on is that both types of scanners will catch items that would be missed by metal detectors.

Physicist Rez said one concern was that the machines are only as good as the people operating them.

Independent scientists can't get access to the scanners for tests, so University of California physicists Leon Kaufmann and Joe Carlson used computer simulations. They focused on the X-ray scanners and discovered it was possible to fool them. "If you play some tricks and make an object with diffuse edges," Carlson said, it can be made harder to see.

Inventor Smith said the paper was only a theoretical analysis because the UCSF researchers never had access to an actual scanner: "They base their calculations on secondhand information, which provides a distorted view of the technology. This is a case of garbage in, garbage out."

But Smith does agree that the 20-year-old X-ray scanner could be improved, which is why Tek84 is working on a new version. The new design is smaller, more sensitive, and sees through shoes so passengers could bypass the hassle of removing them.

Verma adds that in its current form, the X-ray scanner cannot see well through turbans, burkas, or other loose clothing.

National Lab engineer Douglas McMakin said he thought the millimeter-wave scanners did see through loose clothing, including turbans and burkas.

TSA won't go into detail about burka or turban transparency. Asked about the potential for human error, a spokesman said officers were extensively trained.

Is there a cancer risk? The millimeter-wave scanners are unlikely to pose any heath threat unless there's some currently unknown mechanism by which they could interact with the body.

That's not the case with the X-ray scanners, said David Brenner, head of Columbia University's Center for Radiological Research.

The difference is that X-rays are considered ionizing radiation, meaning they have enough energy to knock electrons off atoms and chemically alter DNA and other molecules inside cells.

The wrong combination of DNA changes in a single cell can seed cancer, he said. In the X-ray scanners, most of the energy is deposited in the skin, he said, which might cause a tiny increase in skin-cancer risk.

The millimeter waves used in the other scanner lack the energy to do that.

But Tek84's Verma said the dose from X-ray scanners was negligible, as one would have to go through them 1,000 times to get the equivalent of a single medical X-ray. It's equivalent to a half-hour exposure to natural radiation that comes off rocks, cement, and other parts of the natural environment, he said.

Scientists at the National Institute of Standards and Technology and Johns Hopkins University have evaluated the scanners to ensure they deliver the very small doses claimed.

But the effects of low-level radiation haven't been well studied, said Columbia's Brenner. He agreed that the risk to any individual is tiny - maybe one in 30 million that an X-ray would start a cancer. But tens of millions of people will be going through the scanners.

And some, including flight attendants, could go through them 300 or 400 times a year. Brenner said that because the millimeter-wave technology was available, he saw no excuse for exposing people to X-rays.

Body Scanners At Major Airports

X-ray scanners

Boston

Dulles (Washington)

JFK (New York City)

Los Angeles

Phoenix

Seattle

Radio-wave scanners

Baltimore-Washington

Miami

Newark

O'Hare (Chicago)

Philadelphia

Reagan Nat'l. (Wash.)

San FranciscoEndText

Contact staff writer Faye Flam at 215-854-4977 or fflam@phillynews.com.