Beginning Tuesday, travelers using the Salt Lake City International Airport's Terminal Two have an option to go through a new high-tech unit, instead of the metal detectors, that will scan their bodies and produce images that civil rights advocates worry are so graphic that they will invade their privacy.

The Transportation and Security Administration purchased the technology, developed by New York-based L-3 Communications, for $170,000 and the technology is alongside the eight lines for four metal detectors at the security checkpoint for Terminal Two. Salt Lake City is one of six cities in the United States — including Tulsa, Okla.; Albuquerque; San Francisco; Las Vegas and Miami — in a pilot program using the units. The federal government will analyze passenger data after six months and determine whether to put more units in more airports, Ronald Malin, acting federal security director in Salt Lake City for TSA, said Monday when he introduced the technology to the local media.

How it works: Travelers remove metal jewelry, jackets and shoes and other items they would normally remove for metal detection, then step into a small glass chamber and lift their arms above their heads while sensors scan the body for about three seconds. The sensors are looking for a part of the electromagnetic spectrum that emits energy from the body at wavelengths of about one millimeter, which is longer than light and infrared waves and able to pass through clothing. The energy from a human body is radiated at a higher rate than plastic, metal and ceramic, allowing the unit to reveal those objects.

In a closed-door room about 100 yards away, a computerized image of the person appears on a screen, and a Transportation Security officer looks for items that would be unusual for a traveler to possess. The TSA officers have wireless devices to communicate with each other about whether it's necessary to hold a traveler back in order to check their right back pocket, for example, Malin said.

The media Monday were shown a computer image of an unidentified female who had gone through the scanner. She apparently was wearing a shirt, but the shirt was not clear on the scanner. What was clear was the outline of her body, her bra and her jeans.

Transportation Security officer Staci Bunot, one of about 100 officers trained in the body scanning technology at the Salt Lake airport, said she has never seen bare breasts or genitalia on the computer screen.

Participation in the body scanning is voluntary. Signs tell travelers they don't have to participate and TSA officers are also instructed to tell people it's voluntary. "They'll be able to opt out and go through another lane," Malin said.