Women over the age of 40, regardless of risk level, are advised to get mammograms

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For decades, the mammogram has been the sole recommended, regular imaging procedure for detecting breast cancer. Women over the age of 40, regardless of risk level, are advised to get mammograms -- which use X-rays to look inside the breast -- at least every other year. Women at high risk should start earlier and get tested more often. This has been the standard of care. But in March 2007, two different sources released information recommending that MRI, a magnetic imaging technique, be included in the standard testing procedures for certain women.

The most far-reaching news is a set of guidelines published by the American Cancer Society, which states that women who are in the high-risk group for breast cancer should add MRIs to their regular, yearly testing regimen. MRIs do not replace annual mammograms; they are a supplemental tool that can detect certain tumors that mammograms miss. But breast MRIs have downsides that makes them less ideal for any woman who is not in the high-risk group.

A breast MRI (magnetic resonance imaging), uses magnetism and radio waves to create multiple images of the breast from every possible angle. An extremely high-power magnet creates a magnetic field inside the MRI machine, where the patients is lying for 30 to 60 minutes, and switches the orientation of the field while the machine pulses radio waves into the breast tissue. The radio waves bounce back, or echo, off the various levels of breast tissue, and a magnetized coil surrounding each breast measures the intensity of the echoes. By altering the orientation of the magnetic field throughout the exam, the machine creates views of the breast on every axis -- side to side, front to back, and top to bottom. (See How MRI Works to learn more about the process.)

There are big benefits to MRI. First, there's no radiation involved like there is in a mammogram. And it doesn't require the breast to be flattened, which means it's less uncomfortable than a mammogram. And the big plus is that an MRI can pick up way more abnormalities than a mammogram can. It's unhindered by especially dense breast tissue, which most young women have, and by breast implants, both of which can obscure a tumor on a mammogram. It also images the breast on more planes than a mammogram does, creating a much more detailed view that reveals even subtle differences in breast density, which can be indicators of cancer.

But the biggest plus is also the biggest negative of the breast MRI. It picks up almost everything. If anything looks the slightest bit weird, an MRI is going to reveal it. But there are many, many weird-looking things that show up on breast images that have nothing to do with cancer. Some estimates put MRI false-positives at one in 10, meaning women end up getting a lot of unnecessary cancer terror and invasive biopsies. This is why MRIs are not recommended for women in the average-risk group. Unless there's a pretty good chance you're going to get breast cancer, it doesn't make sense to go through a testing procedure that produces a false positive 10 percent of the time.

And while MRI doesn't use radiation, it does require the injection of a dye to produce greater contrast on the images, and it has trouble picking out certain types of calcifications that can indicate the presence of cancer. Mammograms can see those calcifications. Also, people who are claustrophobic can have problems being in an MRI machine for up to an hour, and the procedure costs about 10 times more than a mammogram. Some insurance plans cover it, but some don't. A woman in the high-risk group would have to spend at least $1,000 a year on a breast MRI if her insurance won't pay for it. Which brings us to another problem: figuring out who's in the high-risk group.