Women with dense breasts who are at a higher risk of breast cancer can get benefits from an annual screening ultrasound, according to a new study.

Lead researcher Dr. Wendie Berg said that the screening -- in addition to mammograms -- helps find breast cancer early. Other steps also help detect problems.

"Significantly more early breast cancer can be found when MRI is performed, even after combined screening with both ultrasound and mammography. However, both ultrasound and MRI increase the risk of false-positive findings," she said.

However, MRI is not for everyone, according to a news release on the work.

"Because MRI is a very expensive test and requires intravenous contrast, it is something we only recommend for screening the approximately 2 percent of women who are known or likely carriers of BRCA1 or BRCA2 gene mutations or have other unusual circumstances that put them at very high risk for breast cancer," Berg said.

Researchers studied 612 women with a high risk. They were screened for a baseline and then given further exams a year and two years later.

In the study, about half of the cancers were seen with mammograms. Adding ultrasound raised the rate to between 70 and 94 percent.

The study also found that supplemental screening with ultrasound or MRI significantly increased the risk of false-positive findings, leading to unnecessary biopsies in some women.

Results of the trail were presented today at the annual meeting of the Radiological Society of North America.