The Federal Aviation Administration has certified a computerized airport explosives detection system that works much like CAT scans in hospitals.

The agency said Tuesday it hopes to begin field testing models of the InVision CTX-5000 for inspecting checked baggage at two airports next year.Congress ordered the FAA in 1990 to develop new devices to protect airports and planes from bombs. It was given a November 1993 deadline to have the devices in commercial operation, but researchers found the task more difficult than anticipated.

"One of the reasons this thing has taken so long is that it's all physics," Bruce Butterworth, the FAA's director of civil aviation security, said Tuesday. "It's very hard to design a system that will reliably detect a very small amount of explosives with a low false alarm rate at a cost that isn't prohibitive."

"Rather than the proverbial needle in a haystack, it's more like trying to find a piece of straw in a haystack, when 400 or 500 haystacks are moving through the belt," Butterworth said.

The devices, developed by InVision Technologies of Foster City, Calif., and Imatron Federal Systems of Burke, Va., resemble in appearance the X-ray machines now used universally to check passengers' carry-on baggage.

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But instead of producing a shadow graph, the tomography system takes multiple views to create cross-sectional images or slices.

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