After months of seeking to allay public fears over privacy, the TSA finds itself increasingly answering another criticism of full-body scanners: Some experts say the machines simply don't work. Critics -- including the Government Accountability Office and private security consultants -- say advanced imaging technology can miss too many threats because it relies on human judgment in interpreting the vague, chalky images. Many critics say the scanners should be used in conjunction with metal detectors, rather than being deployed as a primary screening tool. But the TSA defends the scanners, even as it closes in on its goal of 450 installations by year's end. "No technology is going to be the silver bullet, but this is a significant enhancement," says Robin Kane, assistant administrator for the TSA's Office of Security Technology.