Doctors use magnetic resonance imaging (MRI) to biopsy suspicious breast masses that are seen with MRI, but not with ultrasound or x-ray imaging. MRI-guided biopsy uses a strong magnetic field and radio waves to assist in locating and collecting tissue samples. The risks associated with MRI-guided biopsy are essentially the same as for any biopsy. Although there is no proof that MRI is unsafe to a developing fetus, women who are or suspect that they may be pregnant should talk to their doctors or our medical staff before having an MRI. If you take aspirin or blood thinners, your doctor may tell you to stop several days before the procedure.