Why do some women get MRIs, and others an ultrasound or nothing beyond a mammogram? Should MRI replace the mammogram as a first-line screening tool? And just how accurate are they, anyway? Everything you need to know about magnetic resonance imaging of the breast: its use as both a screening and diagnostic tool.

Q. Susan asks: My latest mammogram showed some abnormalities. I had an ultrasound, and now the doctor wants me to have an MRI. I’ve heard of it, but don’t know what it is…

A. MRI stands for magnetic resonance imaging. Unlike CT scans or mammograms, an MRI doesn’t involve X-rays. Instead, it uses a magnetic field, radio-frequency pulses, and a computer to produce a detailed image of your breast, from the inside out. Since the many images received show very thin “slices” of your breast, they can be put together and viewed from many different angles – unlike the images from an ultrasound or mammogram.

Q. It sounds like an MRI is better than either of those other two tests. Why don’t doctors just start right out with an MRI?

A. Several reasons. First, an MRI is a lot more expensive, complicated, invasive, and lengthy than a mammogram or ultrasound. And it tends to deliver “false positive” results – something that looks like trouble, but isn’t – up to 50% of the time.

If a mammogram and/or ultrasound alone can determine that the lump in your breast needs to be biopsied, then there’s no need to complicate the situation with an MRI.

Q. So, why am I having an MRI on top of those other tests?

A. Because your doctor still isn’t sure what s/he’s seeing, and wants to get as much detail as possible before ordering a biopsy.

A mammogram can “see” a lump, but can’t tell whether it’s solid, or a fluid-filled cyst. An ultrasound can tell if a lump is solid, or a cyst. But it takes an MRI to determine if a solid mass is scar tissue, or a possible tumor, either benign, or cancerous. It can show the specific shape of a mass or lump, with some shapes more likely to be cancerous than others.

MRI can also “see” through dense breast tissue, which is problematic for both mammograms, and ultrasounds. Women with dense breasts (more glandular and connective tissue than fat) are more likely to require an MRI, for both screening and diagnostic purposes, than women whose breasts are predominantly fat tissue.

Finally, an MRI is more accurate in detecting abnormalities in women with breast implants, which can pose problems for mammograms.

If, after looking at the MRI, the doctor is still unable to determine with certainty what s/he’s seeing, then a biopsy may be ordered.

One more fact, before we move on: Women at high risk for breast cancer – such as BRCA1 and BRCA2 carriers – will often have an MRI in combination with a mammogram, or will alternate the two tests on a regular basis. Since their risk is so high, it’s worth the expense, and the risk of false positives, to monitor their situation closely.