Hundreds of hospitals are routinely performing a type of chest scan that experts say should be used rarely, subjecting patients to double doses of radiation and driving up health-care costs. In a double CT scan, patients get two imaging tests consecutively: one without dye and the other with dye injected into their veins. Providence Hospital in Northeast Washington and nearly one of every six hospitals in Virginia were among those performing double scans particularly frequently, according to the most recently published government Medicare data, from 2008. The government is taking a closer look at scans because imaging tests are among the fastest growing procedures in health care. Medicare's Hospital Compare Web site publishes individual hospital rates of double chest scans, along with rates for several other kinds of imaging. Medicare doesn't restrict the use of double scans or penalize those who perform lots of them. "Just making the information public is a fairly strong incentive" for hospitals to examine their rates, says Michael Rapp, director of quality measurement at the federal Centers for Medicare and Medicaid Services.