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 data, from 2008. Nationwide, hospitals performed double scans on 5.4 percent of Medicare patients who received chest CTs in emergency rooms or hospital radiology units where they were referred by their doctor. Those scans totaled 76,781 in 2008. The overall number was certainly higher, as patients with private insurance, Medicaid or no coverage also get double scans, but no one tracks the number.