In the early 1980s, endoscopes were first equipped with a small ultrasound transducer to examine the lining of the upper gastrointestinal tract. Since then, endoscopic ultrasound (EUS) has rapidly developed as an important imaging technique that complements information from other imaging tests such as computed tomography (CT).
EUS uses two basic instrument designs, both of which use an ultrasound probe at the end of an endoscope. The radial imaging device produces a 360-degree, cross-sectional image, while the curved linear array instrument uses Dopler and pulse Dopler ultrasound and can guide fine-needle aspiration, or biopsy, of lymph nodes and tumors.
At various levels of the gastrointestinal tract, EUS is used to examine the multi-layered wall and surrounding structures. In the esophagus, EUS can be used to study the inferior vena cava, azygous vein, aorta, left atrium of the heart, trachea and lung tissue. From the stomach, EUS can inspect the body and tail of the pancreas, aorta, celiac artery, splenic vessels, portal vein, gallbladder and liver. When placed in the duodenum, EUS can be used to study the right kidney, aorta, inferior vena cava, uncinate process of the pancreas, common bile duct, gallbladder and liver. In all locations, regional lymph nodes can be inspected.
EUS is used to:
- Detect small pancreatic tumors when abdominal ultrasound and CT don't provide a diagnosis. This occurs most often when the tumor is less than 2 centimeters in diameter.
- Stage gastrointestinal and pulmonary cancers, including esophageal, gastric, rectal and pancreatic cancers. EUS can accurately determine how deeply a tumor penetrates through the gut wall. Examining the size, shape and ultrasound appearance of adjacent lymph nodes also is helpful to determine whether cancer has spread. In patients with pancreatic cancer, the presence of cancer in the lymph nodes or in adjacent large blood vessels is critical to determine tumor staging and the feasibility of surgical treatment. In patients with esophageal, gastric and pancreatic cancer, it is particularly important to determine whether cancer has spread to the celiac lymph nodes, because such involvement means that the tumor is not curable by surgery alone. In patients with lung cancer, the demonstration of cancerous lymph nodes near the esophagus is also important to tumor staging and treatment decisions.
- Detect common bile duct stones. While stones in the bile tubes leading from the gallbladder to the intestine have traditionally been detected by an invasive test called an endoscopic retrograde cholangiogram (ERCP), EUS is a less invasive method that can detect these stones with equal accuracy.
The ability to do fine-needle aspiration, or biopsy, with EUS has added an important method of determining whether a certain mass is cancerous or whether cancer has spread to lymph nodes. This technique is frequently used to:
- Assess submucosal masses in the gastrointestinal tract.
- Assess enlarged stomach folds that may be involved with cancer deep in the stomach wall and unreachable by surface biopsies.
EUS is becoming more and more important in assessing gastrointestinal disease. However, it is a technically demanding and complex procedure that is best done by an experienced endoscopist.
Mayo Clinic in Florida has offered EUS since 1996. Recent advances have enabled us to move beyond the use of EUS solely as an imaging tool. EUS is now being used as an interventional device for a more detailed, cost-effective and nonsurgical assessment of some diseases. Examples of these new interventional applications include EUS-guided pseudocyst drainage, EUS-guided injection of botulinum toxin for the treatment of achalasia and EUS-guided celiac plexus neurolysis to treat pancreatic cancer pain.
Combining EUS with fine-needle aspiration (FNA), a method of obtaining tissue samples through large-bore needles, has been shown to be a technically feasible, relatively safe method of obtaining cytologic specimens. Right now, EUS-FNA seems to be most useful in diagnosing diseases of the internal organs, such as pancreatic cancer, and determining whether these diseases have spread to other organs, such as lymph nodes.
In addition, EUS-FNA has been shown to be useful in safely and accurately collecting samples of fluid in lungs or in the abdominal cavity for analysis. Finally, EUS-guided fine-needle injection is being evaluated as a possible means of very precisely delivering new cancer treatments to affected internal organs.