CHICAGO — All airline passengers in the U.S. will eventually be required to undergo a full-body scan before boarding planes, just as metal detectors became a standard and accepted part of the screening process at airports decades ago, the federal transportation security chief in Chicago said Monday.

As a body-scanning machine was used to screen passengers for the first time on Monday at O’Hare International Airport, federal and city officials said they expect the airport will receive more body-imaging technology later this year to help address one of the biggest terrorism threats to commercial aviation, suicide bombers on planes.

The Transportation Security Administration plans to send hundreds of the scanners, which cost between $130,000 and $170,000 each, to all major U.S. airports. The scanners use low-dose X-ray to go underneath clothing and display weapons, explosives and other objects that might be hidden on the body, above the skin.

So far, 21 airports are equipped with the units and nine more are slated to receive the scanners soon, officials said. The security agency plans to deploy 450 body scanners to an undetermined number of airports this year.

Kathleen Petrowsky, the TSA director at O’Hare, said she anticipates the body scans — now optional for passengers — will become mandatory in the future to guard against improvised explosive devices being smuggled onto airliners.

Currently, passengers have the option to submit to a physical pat-down and wanding by a TSA officer using a metal detector.

At 19 airports that received the scanners more than a year ago, the devices are now used as a primary means of screening, whereas before they were utilized only to clear up discrepancies when a passenger set off a metal detector or failed a pat-down.

“We expect at some point all passengers will receive a body scan,” Petrowsky said.

Chicago Aviation Commissioner Rosemarie Andolino said she thinks future versions of the 2,200-pound body-scanning machine will be smaller.

As the screening technology is refined, passengers will pass through the body-screening process as they walk through the checkpoint, Andolino said, similar to the way that vehicles equipped with toll-collection transponders travel at highway speed through open-road tolling locations.