IN a shift, the Transportation Security Administration plans to replace the walk-through metal detectors at airport checkpoints with whole-body imaging machines the kind that provide an image of the naked body.
Initially, the machines were supposed to be used only on passengers who set off the metal detectors, to provide them with an option to the customary secondary physical pat-downs and inspections by electronic wand.
But Robin Kane, the agencys acting chief technology officer, said that the initial results from pilot tests at some checkpoints at 19 airports in the United States had been so good that the idea of using the machines as the standard checkpoint detectors made sense. Those results included, he said, positive feedback from passengers.
The plan now is that all passengers will go through the whole-body imager instead of the walk-through metal detector, he said.
Were just finishing some piloting in six airports in the primary screening position, he said. Assuming tests continue to be positive, the machines will eventually be used at most domestic airports.
Still, the use of the equipment has its critics. Bruce Schneier, a security technology consultant, said the body-imaging machines are the equivalent of a physically invasive strip-search.
As to the question of additional X-ray exposure, the agency said the machines emit X-ray doses equivalent to the ambient radiation received in two minutes of airplane flight.
In interviews, agency officials stressed that the technology remained in a test phase. They said that they expected initial contracts to produce the machines to be awarded this summer and that passengers will start seeing the machines at some checkpoints soon afterward. They had no timetable for when the machines would be installed at most airports.
The machines will cost about $100,000 to $170,000 each, depending on the model. They are being developed by the agency as part of an ambitious technology initiative, which also includes advanced X-ray systems for inspecting carry-on bags for weapons and other contraband.
That new X-ray technology may be capable of electronically identifying explosive chemicals, allowing the agency to drop the much-disliked rule that restricts passengers to carrying on liquids or gels solely in containers holding 3.4 ounces or less, packed in a single quart-size zip-top plastic bag.
Passengers undoubtedly would welcome that advance. It is far less clear what the reaction will be once people realize they will be asked to pose in a machine that transmits a naked body image to a screener who, the agency says, is in a remote location and unable to associate the image with the passenger being screened.
In the airports where the whole-body imaging machines are being tested, less than 2 percent of passengers presented with the option of using them are choosing not to, Mr. Kane said.
The development of these machines has been widely known for years. But until now it was assumed that the machines would be used only as an option for the relatively small number of passengers chosen for secondary inspections. The machines excellent performance changed the agenda, Mr. Kane said.
My first reaction is that it will slow down the lines, Mr. Schneier, the security technology consultant and a longtime agency critic, said Monday.
Its almost like an M.R.I. scan, he said. You stand there and hear the machine whirr around you for a few seconds and its done. But thats still a lot longer than walking through the arch and seeing if you beep.
Mr. Kane said that the machines, in tests, have moved people through at about the same rate as the metal detectors.
Its very, very quick; the scan is about two seconds, said Sterling Payne, an agency spokeswoman. Theyll tell you the position to stand in, theres the quick scan, and then you step out of the machine and wait for the resolution, which happens in a separate room in another part of the checkpoint.
Or as Domenic Bianchini, the agencys acting general manager for passenger screening, described it: It looks very closely for anomalies. If theres something on the body that doesnt look like it should be there, thats the red flag to the operator.
The agency says that the images can be adjusted to distort faces and private body parts. The images, which have been described as photolike, will not be stored, and current machines do not have the capability to do so, Ms. Payne said.
Mr. Schneier said he was not so sure. How do we know theyre not going to be storing those images? he asked. Were taking their word for it.