WASHINGTON (CNN) -- Some travelers at key airports in New York and Los Angeles may be put through machines that see through clothing and provide a detailed image of a person's body beginning later this week.

The TSA says the screener who reviews the images is in a booth, unable to see the travelers.

It's the first expansion of the program since the machines were first put to the test in Phoenix, Arizona.

The "whole body imaging" machines have sparked complaints from privacy advocates.

But the Transportation Security Administration says that it has taken steps to protect individuals' privacy and that 90 percent of the travelers in Phoenix preferred the imaging machine to a pat-down.

The millimeter wave machines will be rolled out at John F. Kennedy International Airport in New York and Los Angeles International Airport in California later this week, TSA Administrator Kip Hawley told Congress on Tuesday.

The TSA will be purchasing at least 30 more machines for use at other U.S. airports this year, he added.

The TSA will use two different methods at JFK and LAX as they study the most effective way to use the machines.

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At JFK, passengers sent to secondary screening will be given the option of a pat-down or a trip through the body imager. To protect the traveler's privacy, the screener who reviews the image is in a booth, unable to see the traveler, according to the TSA. The traveler's face is pixilated and the image is not stored, the TSA said.

At LAX, the millimeter wave machine will be located just beyond the checkpoint magnetometers.

Travelers will continuously and randomly be selected to go through the machine. While signs will inform them of the pat-down option, screeners will not announce that choice. But passengers electing not to go through the millimeter wave machine will be given the option of the pat-down.

A TSA spokesman said the agency is still exploring backscatter, another imaging technology in use at Phoenix's Sky Harbor International Airport.

Millimeter wave machines are already in use at airports in Britain, Spain, Japan, Australia, Mexico, Thailand and the Netherlands, as well as at some court and corrections facilities in Virginia, Colorado, Pennsylvania, California and Illinois.

Sky Harbor began using the machines last October.

Hawley also announced Tuesday the TSA is purchasing another 580 multi-view X-ray machines that are used to screen passengers' carry-on bags, bringing the total to 830 machines. The machines give screeners a clearer, more detailed view. Some 600 machines will be deployed by year's end, he said.

The technology is a "powerful platform" that can be adjusted to include software capable of detecting liquid explosives, he said. E-mail to a friend

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