Earlier studies have shown that middle-aged women with dense breast tissue - which contains more connective tissue, glands, and ducts than fat - are five times more likely to develop breast cancer. Scientists want to know why some women have dense breasts while others don't, and why density leaves women more vulnerable to cancer.

Researchers at the Campbell Family Institute for Breast Cancer Research in Toronto studied daughters ages 15 to 30 and their mothers. The daughters and 100 randomly selected mothers received MRIs to measure the percentage of water in the breast tissue, which reflects breast density. All mothers received mammograms. Both tests revealed the same result, showing that MRI is a good test of breast density in younger women.

The amount of dense tissue was also linked to the daughters' height and weight. Taller women, who had higher levels of growth hormone, had denser breasts; heavier women had less dense breasts.

Norman Boyd, lead author of the study, said he is now looking at the link between breast density and cancer risk.

BOTTOM LINE: Breast density in young women, which is linked to a woman's height, weight, and her mother's breast density, can be measured with MRI just as accurately as mammogram but without radiation.

CAUTIONS: All women in this study are Caucasian and it's possible that studies of other racial groups would yield different results. Boyd is planning a similar study with Chinese women.

WHERE TO FIND IT: Lancet Oncology, June 2009