The ultrasound probe is used to locate gall stones which may have migrated into the common bile duct. This occurrence may cause obstruction of the drain shared by the liver and pancreas, which may lead to lower back pain, jaundice, and pancreatitis. Lower digestive tract Echo-endoscopy can also be used for imaging of the rectum and colon, although these applications are lesser known. It is used primarily to stage newly diagnosed rectal or anal cancer. EUS-guided fine needle aspiration may be used to sample lymph nodes during this procedure. Evaluation of the integrity of the anal sphincters may also be done during lower EUS procedures. Respiratory tract An endoscopic ultrasound probe placed in the esophagus can also be used to visualize lymph nodes in the chest surrounding the airways (bronchi), which is important for the staging of lung cancer. Ultrasound can also be performed with an endoscopic probe inside the bronchi themselves, a technique known as endobronchial ultrasound. Technical aspects The quality of the image produced is directly proportional to the frequency used. Therefore, a high frequency produces a better image. However, high frequency ultrasound does not penetrate as well as lower frequency ultrasound so that the examination of the nearby organs may be more difficult. See also Medical ultrasonography Footnotes Diagnostic obstetrics and gynaecology Diagnostic pulmonology Digestive system imaging Endoscopy Medical ultrasonography