I often get asked why can’t a woman just get a breast MRI rather than a mammogram. The imaging tests that we do for breast cancer screening and evaluation of abnormalities have different strengths and weaknesses.
Mammograms are very useful as a screening tool. They can be done quickly and read efficiently by the breast radiologist. They have minimal radiation exposure. They can be done by a mobile coach in locations that are more convenient to patients. They are excellent for identifying abnormal calcium deposits within the breast tissue and for seeing disrupted tissue and masses. They may be less effective in women who have dense breast tissue but the digital techniques have helped some with that.
Ultrasound is a great tool for evaluating a mass or tissue asymmetry found on mammograms. It can distinguish between a benign appearing solid mass, a fluid filled cyst, a mass that is suspicious for cancer, or normal appearing breast tissue. There is no radiation exposure. It is less reliable as a screening tool because it can be dependent on the skill of the physician or technologist doing the procedure. It is possible to miss abnormalities or to mis-interpret normal findings as abnormal. There are studies underway evaluating using an automated version of ultrasound as a screening test but the results are not conclusive and this is not considered ready for standard practice.
Breast MRI is a highly sensitive test that is very dependent on timing, technique, and skill. It is most reliable when done a few days after the onset of menses to offset the effect of hormone changes on the breast tissue. The technique used is critical to getting good images and the multiple images obtained must be carefully interpreted by a radiologist with extensive experience in breast MRI. Since the MRI is so sensitive, if it is normal there is a high level of confidence that there is no pathology. But since it is very sensitive, abnormalities are frequently found that then require evaluation with ultrasound and possibly biopsies – either using the ultrasound or MRI to target the abnormality. For women who are claustrophobic, MRI positioning may be difficult to tolerate.
So these three imaging tests are complementary. They each have benefits and drawbacks. The best approach is to start with mammograms, unless the patient is under age 30, in which case the initial test may be an ultrasound. An experienced breast radiologist will be able to determine which is the best test for any given situation.