Washington ( CNN ) -- One hundred and fifty new full-body scanning machines are set to be placed in airports across the United States as federal authorities work to close security loopholes exposed by the attempted Christmas Day bombing of a U.S.-bound airliner . The Transportation Security Administration ordered the scanners before the bombing attempt for up to $ 170,000 apiece . Plans are also in place to purchase an additional 300 units by 2012 , TSA officials said . Forty body scanners are currently in use at 19 U.S. airports . Use of the technology -- considered overly invasive by critics -- is optional , according to the TSA Web site . Passengers who opt out of the screening receive an alternate form of checking , which includes a pat-down procedure . There are two types of full-body scanners . One uses `` backscatter '' technology to produce an X-ray image of a person . The other uses millimeter-wave technology , which shows a detailed photo negative . TSA officials have attempted to address privacy concerns by promising to blur pictures of passengers ' faces and delete all images produced . One former TSA official told CNN that a body scanner could have stopped Umar Farouq AbdulMutallab , the man accused of trying to blow up a December 25 Northwest Airlines flight from Amsterdam , Netherlands , to Detroit , Michigan . `` I firmly believe in the whole body imaging system as a screening device , '' Charlotte Bryan said . `` I believe that it would have detected that there was something there in that position on his body . It may not have detected it as an explosive , but it certainly would 've raised some flags that would have required further detection and a review either by an officer or further scanning . '' Critics , however , warn that full-body scanners should not be the only answer to continued airport security problems . `` Right now the system is not working as effectively as it needs to be , '' said Geoff Freeman , a representative of the U.S. Travel Association . `` It 's time to hit the pause button and do a wholesale review of what we have today and put in place the policies that will ( give ) America the most secure and efficient travel process . '' CNN 's Mike Ahlers and Sandra Endo contributed to this report .
Forty body scanners are currently in use at 19 U.S. airports . The TSA has purchased an additional 150 machines . Currently use of the technology is optional , according to TSA 's Web site .