Endoscopic Ultrasound (EUS) combines endoscopy and ultrasound to obtain images and information about the digestive tract and the surrounding tissue and organs. Endoscopy refers to inserting a long flexible tube via the mouth or the rectum to visualize the gastrointestinal tract. In contrast, ultrasound uses high-frequency sound waves to produce images of the organs and structures inside the body, such as ovaries, uterus, liver, gallbladder, pancreas, or aorta. Traditional ultrasound sends sound waves to the organ(s) and back with a transducer placed on the skin overlying the organ(s) of interest. Images obtained by traditional ultrasound are not always of high quality. In EUS, a small ultrasound transducer is installed on the tip of the endoscope.

By inserting the endoscope into the upper or the lower digestive tract, one can obtain high-quality ultrasound images of the organs inside the body. Placing the transducer on the tip of an endoscope allows the transducer to get close to the organs inside the body. Because of the proximity of the EUS transducer to the organ(s) of interest, the images obtained are frequently more accurate and more detailed than the ones obtained by traditional ultrasound. The EUS also gathers information about the layers of the intestinal wall and adjacent areas such as lymph nodes and the blood vessels. Other uses of EUS include studying the flow of blood inside blood vessels using Doppler ultrasound and to obtain tissue samples by passing a special needle, under ultrasound guidance, into enlarged lymph nodes or suspicious tumors. A pathologist can examine the tissue or cells obtained by the needle under a microscope. The process of getting tissue with a thin needle is called a fine-needle aspiration (FNA).