WASHINGTON — The Transportation Security Administration is reviving an idea that aims to take one of the biggest hassles out of airport security: removing your shoes at a checkpoint. A dozen companies have designed shoe scanning machines, and the TSA says it plans to buy 100 of the devices by next year. The machines, which find metal weapons and explosives in shoes, didn't pass muster in tests three years ago. The developers of the latest generation of the machines promise better results, and the TSA says the technology will improve security.

Letting travelers keep shoes on "would help checkpoints run more smoothly and allow our officers to focus on other aspects of security," TSA spokeswoman Sterling Payne said. The agency is now reviewing information from the inventors. One company, IDO Security of New York City, has shoe scanners in more than 15 overseas locations, including Ben Gurion International Airport in Israel and airports in China, Rome and Prague, company President Michael Goldberg said. IDO's "MagShoe," which costs $4,400 to $7,000, also is used at a cruise ship terminal in Florida to screen passengers, crew and vendors, Goldberg said.

The TSA began forcing some passengers to remove shoes in late 2001 after Richard Reid tried to ignite explosives in his boots on a U.S.-bound flight. Shoe removal became mandatory in 2006 after a foiled plot to blow up U.S.-bound planes with liquid explosives. The TSA tested a scanner in 2007 at Orlando International Airport but pulled it after seven months because the machine missed too many weapons and bomb parts during tests. The scanner also troubled some passengers because it sounded alarms on shoes containing harmless metal. TSA says travelers often rate shoe-removal as the biggest hassle of checkpoint screening. The new machines range in size from a step stool to a turnstile. Some use electromagnetic fields to detect metal. Others use chemical sensors to detect explosives. The TSA said it wants a machine that finds metal weapons and explosives.

That could be difficult, Goldberg said. IDO's machine uses metal-detection technology that finds powdered explosives containing metal but not plastic explosives. "There is no technology today that can test for all explosives," Goldberg said. Morpho Detection of California told TSA about its scanner that has both metal detection and explosives detection, spokesman Steve Hill said. Another California company, Syagen, told TSA about a prototype that shoots air jets at passengers' feet and lower legs to dislodge explosives particles, company sales director Chris McBee said.