As the federal government investigates America’s chronic disease epidemic, some experts are warning that a popular diagnostic tool may be a contributing factor. CT scans, a go-to for diagnosing everything from tumors to infections to bone fractures, are facing increased scrutiny. Experts say the radiation doses they deliver can vary dramatically — and often exceed what’s necessary. “Many patients still routinely receive radiation doses two or three times what they should,” said Dr. Rebecca Smith-Bindman, a professor at the University of California-San Francisco medical school. “That will lead to cancer in a small percentage of patients, approximately 36,000 cancers every...