WASHINGTON  The Transportation Security Administration plans to install 150 security machines at airport checkpoints that enable screeners to see under passengers' clothes. The installation will vastly expand the use of the controversial body scanners, which can reveal hidden bombs and knives. But the devices have been labeled as intrusive by some lawmakers. The House of Representatives in June overwhelmingly passed a measure that would restrict their use by the TSA to passengers flagged by other types of screening, such as metal detectors. The measure is pending in the Senate. TULSA: Passengers try out body scan TSA spokeswoman Kristin Lee said the machines are "critical" to stopping terrorists with homemade bombs that may elude metal detectors. The agency hasn't decided which airports will get the machines, Lee said. The $100,000 scanners shoot low-intensity X-rays that penetrate clothing, bounce off a person's skin and create images that show solid objects as dark areas. The TSA machines have privacy additions to create images that look like etchings. Screeners view them on a monitor in a locked room near a checkpoint and delete them immediately after viewing. "Body imaging is a total invasion of privacy," said Rep. Jason Chaffetz, R-Utah, who proposed the restriction. "You don't need this kind of scrutiny." Although the machines use X-rays, a 2003 report by the National Council on Radiation Protection & Measurements, which Congress created to develop radiation guidelines, said people can safely be scanned by the machines up to 2,500 times a year. "Imaging technology is safe," Lee said. The TSA used $25 million from the federal stimulus package to buy the scanners from California-based Rapiscan Systems. The agency is using an additional $22 million to buy 500 upgraded machines that scan bottles for liquid explosives. The TSA has been testing scanners since early 2007, mostly on passengers who set off a metal-detector alarm and are taken aside for additional screening. The new scanners will be installed beginning early next year and will be used in place of metal detectors at checkpoints. Passengers may choose to avoid the scanners and be screened by a metal detector, but those who do will be pulled aside for a pat-down, Lee said. American Civil Liberties Union lawyer Christopher Calabrese said using the scanners in place of metal detectors "is unquestionably a step in the direction of having these machines be mandatory." Guidelines: You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. Read more