Magnetic resonance imaging (MRI) uses powerful magnetic fields and radio waves that are fed into a computer to generate three-dimensional images of breast tissue.
MRI highlights the differences in water content and blood flow between tissues, and because tumors tend to have more blood flow than surrounding tissues, MRI is an effective tool for visualizing tumors. In some instances technicians administer contrast agents to patients in order to further improve image quality.
MRI does not replace mammography or ultrasound imaging of breasts. Instead it provides a powerful supplementary tool for detecting and staging breast cancer. In addition to its role as a diagnostic tool, researchers are investigating whether breast MRI may be useful in screening younger women at high risk of breast cancer.
For example, MRI is particularly good for detecting very small tumors and therefore can be used after a suspicious spot has been detected by mammogram or ultrasound to determine whether the cancer has spread further in the breast or into the chest wall.
MRI is also especially useful for detecting tumors in women with breast implants (which can interfere with mammogram rays), and in women with dense breast tissue. This last point makes MRI a particularly desirable addition for women at high risk of developing breast cancer who are more likely to begin screening at a younger age, when breast tissue tends to be denser.
The best MRI technique involves the use of a special “breast coil”. During an MRI, you lie still and are moved in and out of a narrow tube as the machine creates images of your body. If you’re claustrophobic, being confined within an MRI machine for up to an hour can be difficult. Some facilities have an open MRI machine to avoid this problem, or you may be given a mild sedative.
The value of MRI for breast cancer detection remains uncertain. Some doctors believe MRI can distinguish a breast cancer from normal breast gland tissue better than other techniques. But MRI is expensive and requires highly specialized equipment and highly trained experts. Relatively few MRI centers exist, especially outside of major cities. Even at its best, MRI produces many uncertain findings. Some radiologists call these “unidentified bright objects”, or UBOs. MRI also cannot detect calcifications. Finally, MRI can dislodge certain metal devices, such as pacemakers, in some people.
When is MRI useful?
It is unlikely that MRI will be used as a general screening tool for breast cancer. It may, however, prove useful in:
- evaluating a woman who has a palpable mass that isn’t visible with ultrasound or mammography
- assessing a lesion in the densely glandular breast of a young woman
- screening a young woman who is at high risk for cancer because of a significant family history of breast cancer or an abnormal breast cancer gene
What are the advantages of MRI?
- MRI is sometimes used successfully in women who have breast cancer cells in an underarm lymph node, but have no breast mass that doctors are able to feel or see on a mammogram. In these cases, where mastectomy is typically recommended, MRI can help find the precise site of the cancer’s origin within the breast. Finding the cancer’s precise origin can expand a woman’s treatment options from only mastectomy to include lumpectomy plus radiation.
- MRI can help determine if a cancer is limited to one area of the breast, or if it is “multicentric” and involves more than one area. Knowing this affects treatment choices, since mastectomy is necessary for multicentric disease. This is particularly useful for women with invasive lobular cancer, which has a tendency to be diffuse or multicentric.
- MRI is good for looking at scar tissue. It can evaluate a significant change in the lumpectomy site.
- MRI scanning can detect leakage from a silicone-filled breast implant, since it easily distinguishes silicone gel from surrounding normal breast and chest wall tissues.
- In the case of metastatic breast cancer, MRI can evaluate other parts of the body. A woman who has progressive back pain, or who develops new weakness or numbness in the arms or legs (not just hands or feet), can have an MRI scan of her back. The scan can help identify serious conditions such as the possible presence of a spinal tumor or brain metastasis.