The Transportation Security Administration (TSA) is running a test at a North Carolina ferry terminal of a 21-foot-high arch-like machine that shoots low-intensity X-rays at cars as they pass through. The photos show whether explosives or drugs might be in the car.

The technology, called backscatter X-ray, is in use at several airports to screen passengers. Privacy advocates have denounced scanning people as invasive because the X-rays can see through clothes.

Melissa Ngo, a Washington, D.C., lawyer who focuses on privacy issues, says using the technology for cars could pave the way for ''Big Brother'' government spying.

"If this technology ends up being deployed widely, it seems to be another step toward a society where you need to accept surveillance in every part of your life,'' Ngo says.

TSA Assistant Administrator John Sammon said motorist privacy won't be invaded because taking X-rays of cars ''is a fairly non-intrusive way of being able to inspect vehicles that are coming in'' to an airport. At many airports, cars are currently stopped at random and searched by authorities.

Sammon said the TSA has made no decision about using the X-ray portals at airports. The four-week test at a small ferry terminal in Cherry Branch, N.C., will gauge public reaction and the machine's ability to find bombs, Sammon said.

Searches escalated last summer after two men with ties to al-Qaida drove an SUV into entrance doors at Glasgow Airport in Scotland, igniting a fire. Sammon said the test is not related to the Glasgow terror attack.

The TSA has no intelligence pointing to a potential car bombing at an airport, Sammon said, adding that the X-ray machine could be useful if there were such a threat. ''Rather than stopping all vehicles or doing random checks, you could run vehicles, especially large-panel trucks, through the machine,'' Sammon said.

Ferry officials say the X-ray portal has intrigued but not alarmed motorists lining up for the 20-minute ferry ride from Cherry Branch across the Neuse River to Minnesott Beach on North Carolina's coast. ''I am not aware of any complaints,'' said David Lewis, security coordinator for the North Carolina Ferry Division.

Similar machines are protecting military bases in the United States and in Afghanistan, said Joe Reiss, head of marketing for manufacturer American Science and Engineering. The machines, first marketed last year, cost $1.2 million to $1.7 million.

Here's how the car scanner works

» Car drives slowly under arch-like portal.

» Portal takes X-ray images of top and two sides.

» Screener views images for large organic materials such as bombs or narcotics.

» Screening takes about 30 seconds.

Source: American Science and Engineering