The Transportation Security Administration's new rules for screening passengers with its controversial full-body scanners - which were quietly changed just before the busy holiday travel season - represent a significant policy reversal that could affect your next flight. Getting checked by the TSA's advanced-imaging technology used to be entirely optional, allowing those who refused a scan to be subjected to a pat-down. In fact, many observers thought the agency installed the 740 body scanners in 160 airports with an understanding that no one would be forced to use them, ever. But on a Friday in late December, the TSA revised its rules, saying an "opt out" is no longer an option for certain passengers. "The TSA is going back on its word," says Fred Cate, a law professor at Indiana University and prominent TSA-watcher. "The scanners were sold to Congress and the public on the promise that they were optional, but for at least some people, that is no longer the case." In previous court filings, the agency offered written assurance that the scanners were optional. Based on the agency's statements, a federal appeals court affirmed the legality of using the full-body scanners as long as fliers were given a choice.