ATLANTA — Up to 1.4 million U.S. women — those with an unusually high risk of developing breast cancer — should get annual MRIs as well as mammograms, the American Cancer Society advises in new guidelines.

And a new medical study suggests that all women newly diagnosed with breast cancer should get MRIs, too. The scans revealed cancers in the opposite breast that were missed by ordinary mammograms in 3 percent of these cancer survivors.

The study came out after the cancer society developed its guidelines, which are the first to recommend MRIs for screening women who show no signs of cancer.

The guidelines are directed at symptomless women age 30 and older who have a mutation in the BRCA1 or BRCA2 genes; those who were treated for Hodgkin's disease; or those with a strong family history of the disease, such as women with two or more close relatives who had breast or ovarian cancer or who have a close relative who developed breast cancer before age 50.

As many as 1.4 million women fall into the affected group, according to an American Cancer Society estimate.

Doctors usually screen for breast cancer using mammography, an X-ray technique that can spot dense masses like tumors.

MRI, or magnetic resonance imaging, makes more detailed images with a magnet and radio waves — but without radiation. MRIs are better at showing increased or abnormal blood flow in the breast, a sign of early cancers not visible on a mammogram. They also are better than mammograms at detecting cancer in women with dense, non-fatty breasts.

But MRI screening is not being recommended for most women. One reason is the test's error rate, which can lead to unnecessary biopsies.

Another is the high cost. While a mammogram costs roughly $100 to $150, an MRI can cost $2,000 or more at some medical centers, experts said.

"It's very, very expensive," said Robert Smith, the cancer society's director of cancer screening. Many insurers cover MRI screenings, but not all do, Smith and others said.

Insurers generally follow government guidelines, but the cancer society guidelines could prove influential, according to experts.