More and more physicians are investing in their own imaging equipment. But when a doctor stands to make money on each MRI he or she orders, it doesn't take a brain surgeon to figure out that they might be inclined to order too many scans.

Patients with back problems whose orthopedic surgeons referred them for an MRI were much more likely to have their spinal lumbar scan come back clean—indicating that the test might not have been necessary—if their doc had a financial stake in the equipment being used, than if he or she didn't, according to new findings that were presented this week at the Radiological Society of North America's annual meeting in Chicago.

"It is important for patients to be aware of the problem of self-referral and to understand the conflict of interest that exists when their doctor orders an imaging exam and then collects money on that imaging exam," said Ben Paxton, a radiology resident who led the study at Duke University Medical Center in a prepared statement.

Of 250 spine lumbar MRIs ordered by orthopedic surgeons who had financial interest in the imaging equipment, 106 scans came back negative—that is, without serious abnormalities. Of the 250 lumbar MRIs ordered by orthopedic surgeons in the same area who would not see an extra penny from the scans, 57 came back negative.

Scans that did find problems seemed to be of equal severity in both groups, suggesting that the doctors should be referring roughly equal portions of patients for MRIs.

The two patient groups did show one interesting difference, however. Doctors with financial interest in the equipment "were much more likely to order MRI exams on younger patients," Paxton said. The mean age for patients getting an MRI from these physicians was about 50, whereas the disinterested docs' patients were closer to 57. That means the physicians with financial incentives might have different threshold altogether for recommending an MRI, he suggested.

The new findings suggest that studying these detailed correlation can help shape more effective health care cost cuts. Medical imaging overall cost Medicare some $14.1 billion in 2006—about twice as much as in 2000, according to a 2008 U.S. Government Accountability Office report. About $3.9 billion of that was from physicians who referred patients for imaging in their own offices, according to a July Journal of the American College of Radiology paper, co-authored by Ramsey Kilani, also of Duke, who worked on the new study as well.

"Attempts to date at reducing health care spending on medical imaging have come in the form of across-the-board cuts that threaten to reduce access to vital imaging services," Kilani said. "We believe patients would be better served if we instead eliminated underlying drivers of unnecessary imaging spending." Reducing doctor ownership of these scanners might be difficult in the short-term. So as these income-generating machines proliferate in physicians' portfolios, the next time your doctor orders an MRI, it might be worth asking about the financial arrangement—or getting a second opinion from a doctor who hasn't invested in equipment.