Few women have magnetic resonance imaging exams or MRIs to screen for breast cancer, but MRIs are often used to help diagnose cancer after it has been detected through a mammogram or a physical.

The sensitivity of the MRI helps doctors determine the size and position of the tumor prior to surgery -- but new evidence has suggested this extra preparation may not be necessary.

Researchers in the UK compared two groups of women. Both groups initially had a clinical exam, ultrasound imaging, and a needle biopsy. But half the women also received an MRI.

Doctors expected that the MRI would give precise information about the tumors so they could be removed in a single operation and the women would need fewer follow-up surgeries.

However, results showed the surgeons missed parts of the tumors even with the MRI images -- 19 percent of women in both groups had to have additional operations.

Despite these findings, some experts caution that MRIs may still be worth the added costs because they detect a small percentage of cancers that other tests can miss.