Ultrasound uses sound waves rather than x-rays to produce images of the tissues inside the breast. A breast ultrasound can show all areas of the breast, including areas harder to study with a mammogram.
A breast ultrasound is used to see whether a breast lump is filled with fluid (a cyst) or if it is a solid lump. An ultrasound does not replace the need for a mammogram, but it is often used in conjunction with mammograms to help check dense breasts and to check areas of interest demonstrated on a mammogram.