Endoscopic ultrasound (EUS) is a minimally invasive procedure to assess digestive (gastrointestinal) and lung diseases. It uses high-frequency sound waves to produce detailed images of the lining and walls of your digestive tract and chest, nearby organs such as the pancreas and liver, and lymph nodes.When combined with a procedure called fine-needle aspiration, EUS allows your doctor to sample (biopsy) fluid and tissue from your abdomen or chest for analysis. EUS with fine-needle aspiration can be a minimally invasive alternative to exploratory surgery.EUS techniques are also used in certain treatments, such as draining pseudocysts.