After being diagnosed with breast cancer, a breast MRI may be performed to determine: - How large the cancer is and whether it involves the underlying muscle. If there are other cancers in the same breast and whether there is an unsuspected cancer in the opposite breast. - If there are any abnormally large lymph nodes in the armpit, which can be a sign the cancer has spread to that site. - Further evaluating hard-to-assess abnormalities seen on mammography Sometimes an abnormality seen on a mammogram cannot be adequately evaluated by additional mammography and breast ultrasound alone. In these rare cases, MRI can be used to definitively determine if the abnormality needs biopsy or can safely be left alone.