"One of the thoughts for the change in the rise of imaging is a change in the incentives," Dr. Rebecca Smith-Bindman, a professor of radiology and biomedical imaging at UCSF, told FoxNews.com.  "So we wanted to study patients see if there was an increase when there weren't an incentives, and the rate of growth is very similar in all the other settings.  It suggests that financial factors may play a role, but there are other factors as well."

To get their findings, Smith-Bindman and her colleagues studied close to 2 million patients every year for the course of 15 years.  Each patient was enrolled in one of six integrated health care systems - including HMOs.

At the beginning of the study, the researchers found that 50 CT scans were obtained for every 1,000 adult patients.  At the study's conclusion, the number increased to 200 CT scans for every 1,000 patients - a 20 percent increase.