President Obama's push to deploy body-imaging scanners at airports worldwide will cost U.S. taxpayers roughly $3 billion over eight years, congressional investigators report, but it is unclear that the controversial devices would have caught an alleged al-Qaida terrorist who tried to blow up a Detroit-bound jetliner with explosives hidden in his underwear. The administration has cited the Christmas day attack by Umar Farouk Abdulmutallab in pushing to double its planned deployment, to 1,800 scanners, at U.S. airports by 2014, and to encourage foreign governments to use the same new technologies at airports that send flights to the United States. The benefits and limitations of the machines will be discussed Wednesday afternoon at a hearing before the House Homeland Security Committee. 'In the never-ending race to protect our country, we have to stay one step ahead of a nimble adversary. That's what these steps are designed to do,' Obama said Jan. 8 in announcing increased aviation security and screening measures. The machines create images outlining the unclothed human body by bouncing X-rays or radio waves off skin or concealed objects. However, security experts say the 'advanced imaging technology' has limits: 'Backscatter' rays can be obscured by body parts, might not readily detect thin items seen 'edge-on' or objects hidden inside the body, and require a human operator to decide whether to conduct additional questioning or a physical search. 'While officials said (the scanners) performed as well as physical pat downs in operational tests, it remains unclear whether the AIT would have detected the weapon used in the December 2009 incident,' the Government Accountability Office, Congress's audit arm, said in testimony prepared for the hearing. Abdulmutallab allegedly concealed 80 grams of explosive powder in a pouch sewn into his underwear. GAO official Steve Lord is expected to tell the committee that investigators are still reviewing TSA operational tests conducted at the end of last year. 'While GAO recognizes that TSA is attempting to address a vulnerability exposed by the December 2009 attempted attack, a cost-benefit analysis is important as it would help inform TSA's judgment about the optimal deployment strategy for the AITs, and how best to address this vulnerability,' the prepared testimony states. The audit agency said TSA estimates each unit costs about $170,000, meaning it would cost about $300 million to buy 1,800 units, enough to cover about 60 percent of screening checkpoint lanes at the highest-priority commercial airports. Each scanner requires three people to operate. Based on the administration's request for $219 million to hire 3,550 TSA staffers next year alone, GAO estimates it will cost $2.4 billion overall to staff the machines over eight years. A TSA spokeswoman said the agency has already conducted a cost analysis and determined that scanners are better than existing alternatives, including metal detectors and machines that check swabs of people's hands or belongings for traces of explosives. As such, TSA said, the machines increase the odds that security officials can detect anomalies in a fraction of the time and inconvenience as pat-down searches. 'While there is no silver-bullet technology, AIT is very effective at detecting metallic and nonmetallic threats on passengers, including explosives,' TSA said in a statement. 'It's one part of our multi-layered strategy to minimize risk, deter future attacks and protect the traveling public.' 'There is, of course, no foolproof solution,' Obama said in January, saying only that the new scanners might have stopped Abdulmutallab. Privacy groups have opposed the machines, saying they amount to electronic strip searches whose images are prone to abuse. The Inter-Agency Committee on Radiation Safety, a group that includes the European Commission, International Atomic Energy Agency and the World Health Organization, suggested earlier this year in an internal report that, although the radiation dose from scanners is extremely small, pregnant women and children should not be subjected to scanning. Before the Dec. 25 attack, lawmakers in Europe and the United States previously voted last year to limit use of the scanners. 