(CNN) -- The federal government is starting to deploy full-body scanning machines to 11 airports across the United States, Homeland Security Secretary Janet Napolitano announced Friday.

Forty body-imaging machines already have been put into use at 19 airports nationwide as part of a field test, according the Department of Homeland Security. The Transportation Security Administration expects to deploy 450 units by the end of this year.

"By accelerating the deployment of this technology, we are enhancing our capability to detect and disrupt threats of terrorism across the nation," Napolitano said in a statement.

The first of the new units are being installed Friday at Boston's Logan International Airport, according to a DHS statement.

The list of other airports set to receive the scanners by the end of summer includes Chicago O'Hare International, Fort Lauderdale-Hollywood International, Cincinnati/Northern Kentucky International, Mineta San José International, Los Angeles International, Port Columbus International, Oakland International, San Diego International, Kansas City International and Charlotte Douglas International.

The imaging machines are being funded through the Obama administration's $862 billion economic stimulus plan.

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.

Passenger privacy is maintained during the scannning process by blurring all images, deleting images after they are viewed and placing the screener viewing the images in a remote location, according to DHS officials.

Acting TSA Administrator Gale Rossides testified before Congress on Thursday that the machines will not significantly slow the passenger screening process, saying it will be done at the same time as carry-on baggage screening.

The TSA has spent years testing full-body imagers. Plans to deploy them this year were given added urgency after the arrest of a Nigerian man, Umar Farouk AbdulMutallab, who has been accused of attempting to detonate an explosive sewn into his underwear on a December 25 flight.

Field testing of full-body scanners already is under way at the following 19 airports:

• Albuquerque International Sunport Airport

• Hartsfield-Jackson Atlanta International Airport

• Baltimore/Washington International Thurgood Marshall Airport

• Denver International Airport

• Dallas/Fort Worth International Airport

• Detroit Metro Airport

• Indianapolis International Airport

• Jacksonville International Airport

• McCarran International Airport

• Los Angeles International Airport

• Miami International Airport

• Phoenix Sky Harbor International Airport

• Raleigh-Durham International Airport

• Richmond International Airport

• Ronald Reagan Washington National Airport

• San Francisco International Airport

• Salt Lake City International Airport

• Tampa International Airport

• Tulsa International Airport

CNN's Mike Ahlers contributed to this report.