What’s the Difference Between a Breast MRI and a Mammogram?

Mammograms are the most common breast cancer screening tests currently available, but they aren’t the only test used for detection and diagnosis. Breast MRI scans also play a role in locating and treating breast cancer (and so do breast ultrasounds, but that’s a subject for another blog post).

It’s not that one imaging modality is better than another; mammograms and MRI scans simply have different strengths. Doctors might even order both. Here’s the difference between these commonly confused procedures, along with a few reasons doctors might prefer one over the other.

How Mammograms Work

A mammogram is an X-ray of the breast, which radiologists use to identify changes in tissue that may be signs of cancer. There are two types of mammograms: screening and diagnostic.

Screening mammograms are routine check-ups for women who have not been diagnosed with cancer. Doctors use diagnostic scans, on the other hand, when patients report breast issues, or when they spot an abnormality. Diagnostic scans are also a screening method for women who have already been treated for breast cancer.

Mammogram machines use X-rays with a relatively low radiation content to create images, resulting in a high-quality picture and very little radiation exposure. A two-plated machine flattens breast tissue, allowing the low-dose X-ray to more easily travel through the body.

In addition to the standard mammogram, digital and 3D scanning options are now more readily available to women. Digital scans work the exact same way as standard mammograms, but save the results as computer files, instead of as large, printed sheets of film.

3D scans create a more in-depth image of breast tissue. Instead of the two X-ray images involved in a standard mammogram, a 3D scan takes many images and layers them together to make a clear 3D image of the breast. This scan creates clearer pictures of the tissue, making it easier for doctors to analyze the results.

How Breast MRI Scans Work

When a medical professional diagnoses a woman with with breast cancer, they might order a breast MRI to determine the size and exact location of the tumor. Doctors also use these scans to make sure the other breast is free of tumors.

Doctors don’t usually use breast MRIs for screening because these scans are not as effective as mammograms at identifying cancers. Also, MRIs frequently reveal shadows that don’t turn out to be cancer — a “false positive.” Medical teams have to investigate false positives, creating the need for unnecessary extra tests, screenings, or biopsies.

MRIs use strong magnets instead of radiation to create pictures of the body. The MRI machine takes many photos from different angles to create a comprehensive image. Similarly to mammograms, breast MRI scans require special equipment. A machine must have dedicated breast coils to be able to perform a breast MRI.

During a breast MRI, patients lay face down on a the MRI table, with their breasts hanging down into an opening in the table. There is no breast compression, making the scan painless. The table then slides into the machine, where the patient must stay very still for the duration of the scan (45-60 minutes). Doctors occasionally use an injection of contrast to create clearer images.

How Do Doctors Choose Between Mammography and Breast MRI Scans?

As mentioned, doctors prefer mammography for screening purposes; they only order MRI scans of the breast in more high-risk scenarios. The main reason this preference persists is scientific: Mammograms remain the only diagnostic imaging modality clinically proven to be associated with lower mortality from breast cancer.

Besides, there’s still the issue of false positives. MRI scans are so sensitive that they reveal benign lesions along with malignant tumors, and we can’t reliably distinguish one from the other without a biopsy. The fear is that replacing mammographs with MRI scans of the breasts will lead to many unnecessary biopsies, with all the anxiety that comes with them.

Today, radiologists use every tool at their disposal. Both mammography and breast MRI scans save lives, and that’s one important thing that they do have in common.