Scans of people’s knees are less likely to reveal a problem when the referring doctor has a financial stake in the imaging center or the equipment used, suggesting some tests may be unnecessary, according to a new study published in the journal Radiology and summarized by Reuters Health.

Researchers concluded that When doctors have a financial interest in the imaging facility, their patients are 33 percent more likely to get a test result that shows nothing wrong, compared to patients of doctors with no financial interest. “It does raise the questions: Are these studies being performed unnecessarily? Are these machines being over utilized because of an unconscious bias?” Dr. Matthew Lungren, the study’s lead author who did the research while at Duke Medicine in Durham, North Carolina, said.

In medical circles, it is known as “self referral” when doctors send patients to get images or scans taken at centers they partially own.

Lungren and his colleagues reviewed 700 MRIs of knees taken between January and April 2009 at a single imaging facility and ordered by two groups of doctors practicing in the same geographic area. One set of doctors had a financial stake in the imaging facility and the other did not. The patients in each group were similar to each other in age and the doctors in each practice also had similar training.

Of the 350 MRIs ordered by the group with a financial interest in the imaging equipment, 117 of the tests were negative for a problem with the knee. That compared to 88 negative results among the 350 MRIs ordered by the group without a financial interest in the MRI machines.

“There are a lot of possible explanations for this but the bottom line is that there is a significantly higher number of negative studies coming out of the one specific group,” Lungren said. He cautioned the new study cannot show that owning a stake in an imaging center or its equipment caused doctors to order unnecessary tests.

For example, Lungren and his colleagues write that the doctors in the group that owns part of the imaging center and its equipment may have a culture of ordering more tests, compared to the group without a stake in the imaging centers or equipment.

But Dr. David Levin, professor and chairman emeritus of the Department of Radiology at Thomas Jefferson University in Philadelphia, said he is not surprised by the results. “This whole issue of self-referral and imaging has a long history,” he said. “Every study that’s ever been done shows self-referring physicians are going to do more imaging than physicians who refer patients to hospitals or imaging centers.”

An MRI of the knee can cost between $700 and $1,000.