Ultrasound uses sound waves with frequencies above those humans can hear. A transducer sends sound waves traveling into the body which are reflected back from organs and tissues, allowing a picture to be made of the internal organs. Ultrasound can show tumors, and can also guide doctors doing biopsies or treating tumors.
Ultrasound image of the liver; dark areas by arrows show possible tumors. Image courtesy of Dr.Thomas Shawker, National Cancer Institute.