Nearly half of women with a high risk of breast cancer who are eligible for an MRI to look for tumors refuse the service, according to a new study in the journal Radiology.

Researchers from Johns Hopkins University found that 42 percent of the women pass.

"Given that MRI is promoted as a very sensitive test to identify early breast cancer, we were surprised that barely half of women at increased risk for breast cancer would undergo MRI even when offered at no cost," said Dr. Wendie A. Berg. "This suggests the need for alternative methods, such as ultrasound, to help screen women at increased risk for breast cancer."

A news release on the work said that other studies have shown that MRI can find tumors missed by traditional mammography and ultrasound.

The authors said that about one-quarter of women who refused the MRI said it was because they do not like the enclosed spaces. Other reasons included not wanting an IV injection, a lack of time or financial concerns.

The results were based on a study of 1,215 high-risk women in another study. They were offered the MRI. About 58 percent agreed to it, though only 51.6 percent actually had it done.

"Screening with ultrasound helps to show small invasive cancers not seen on mammography and can be used to supplement mammography in high-risk women who cannot undergo MRI screening," Berg said. "Women who are at increased risk due to intermediate family history or dense breast tissue but do not meet the recommended risk level for MRI can also consider screening with ultrasound in addition to mammography."