WHAT IS A BREAST ULTRASOUND?
Breast ultrasound is a noninvasive medical test that uses sound waves (not x-rays) from a hand-held unit called a transducer to produce pictures of the inside of the breast.
An ultrasound test may be useful if your mammogram shows a mass, or if a lump is felt during a breast exam that may or may not be seen on a mammogram. A breast ultrasound examination is not considered a screening test, nor does it replace a mammogram.
The ultrasound machine can create an image that often allows the radiologist to distinguish between a fluid-filled cyst and a solid mass. Mammograms do not make this distinction, though they are better than ultrasounds at detecting microcalcifications, which can be an early sign of breast cancer.
https://www.med.unc.edu/radiology/breastimaging/services/breast-ultrasound/