Washington (CNN) -- The acting head of the Transportation Security Administration gingerly answered questions Thursday about whether new full-body imaging machines will detect concealed explosives like that allegedly placed in the underwear of the Christmas Day bomber.





The deployment of 150 of those machines in front-line use at 11 U.S. airports is to be announced Friday by Homeland Security Secretary Janet Napolitano, TSA officials say.





"Would this machine have detected a bomb on that person?" Rep. Hal Rogers, R-Kentucky, asked Gale Rossides, the agency's acting administrator.





"Without going into the specifics of that because of the ongoing criminal investigation, I can tell you that the experience we've had both in the labs and in our [field tests], our officers are identifying objects on the body that are comparable to what that threat was," Rossides said.





"Every time?" Rogers asked.





"Our officers are doing a very good job," Rossides answered.





"Every time?" Rogers pressed.





"I'd have to get back to you, but you know we have very, very good measures in place for evaluating our officers," she said.





After several unrelated questions, Rogers asked one more question about the machine's capabilities.





"Is this machine the end-all?" he asked.





"This machine gives us an increased detection capability that is significantly greater than what we have at the checkpoint today," she said, adding that her agency is "driving the manufacturers" to improve the technology.





Rossides testified that the full-body imagers -- which see through clothes, producing a ghostly image of a traveler's body -- will not reveal objects hidden in body cavities. But she said imagers are better at detecting concealed items than a pat down.





"Today, we do not do a full-body pat down that goes into the sensitive parts of the body where that [Christmas Day bomb] was secreted," Rossides said.





On Friday, Napolitano will announce the first deployment of whole-body imagers as a standard part of checkpoint equipment. Though 40 machines have been deployed at 19 airports nationwide in field tests, Friday's announcement will mark the official beginning of the program, TSA officials said.





The new machines will be deployed this weekend in Boston, Massachusetts, and Chicago, Illinois, the agency said Wednesday. It will reveal the remaining airports Friday.





The 150 new machines were purchased last fall with federal stimulus money. The agency hopes to deploy nearly 500 units by the end of the year.





Under existing protocols, full-body scans are optional at airport checkpoints. Travelers who decline the scans are funneled to a location where they may be given a pat-down and subjected to other tests, such as swabs that can detect minute traces of explosives on hands or luggage.





The TSA said most passengers prefer a body scan to a pat-down. But others have objected to the body scans, calling them electronic strip searches.





Rossides testified that the machines will not significantly slow the passenger screening process, saying it will be done at the same time as the carry-on baggage screening.





The agency has spent years testing whole body imagers. Plans to deploy them this year were given added urgency after the arrest of a Nigerian man, Umar Farouk AbdulMutallab, who was accused of attempted to detonate an explosive sewn into his underwear aboard a flight December 25.
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question: What does the agency hope to deploy?, answer: full-body imaging machines | question: what is Homeland security chief going to announce?, answer: The deployment of 150 of those machines in front-line use at 11 U.S. airports | question: What will homeland security announce, answer: The deployment of 150 of those machines in front-line use at 11 U.S. airports | question: what are they using to detect concealed explosives?, answer: full-body imaging machines | question: how much will it affect?, answer: not significantly slow the passenger screening process, | question: how many sich imagers is the agency hoping to deploy?, answer: 500