Understanding Endoscopic Ultrasound and Fine Needle Aspiration
Since its initial report of use in 1992 as an imaging modality, endoscopic ultrasound-guided fine needle aspiration (EUS-FNA) has been incorporated into the diagnostic and staging algorithm for the evaluation of benign and malignant diseases of the gastrointestinal tract and adjacent organs. This constitutes a major breakthrough in the endoscopic field, which has gradually transformed EUS from a pure imaging modality into a more interventional procedure. EUS permits the collecting of samples, providing definitive cytological and histological evidence of malignancy, which has strongly contributed to changing EUS from a subjective, operator-dependent procedure into a more objective one.
EUS is a technique that combines and modifies techniques of gastrointestinal endoscopy and ultrasonography. It uses sound waves known as ultrasound during an endoscopic procedure to look at or through the wall of the gastrointestinal tract. The common indications for EUS-TA (tissue acquisition) include diagnosis and staging of pancreaticobiliary, esophageal, gastric and rectal malignancies and lung cancer, along with evaluation of GI subepithelial lesions and lymphadenopathy. In addition, EUS-TA can be used for numerous nonmalignant processes and mediastinal lesions such as tuberculosis, sarcoidosis, abscesses and cysts.
EUS is a diagnostic modality. The concept behind endoscopic ultrasound is to diminish the distance between the ultrasonic source and the organs to be imaged. An ultrasound transducer is incorporated into the tip of an endoscope. While endoscopically visualizing the lumen, the probe can be guided and placed adjacent to the area of disease, thus avoiding bones, tissue and air-filled structures, all of which limit sound wave imaging clarity.
The short distance between the ultrasonic source and the target permits the use of high frequency, high resolution sound waves which, due to their shorter penetration depth, are not suitable for transcutaneous ultrasonography.
Finding an abnormality with regular endoscopy is like seeing the tip of an iceberg. Endoscopy shows only the inner surface of the digestive tract and cannot show the abnormality beyond the visible surface. But in the same way a ship’s sonar can depict the whole iceberg under water, the high frequency of EUS reveals the full extent and nature of abnormalities, including information that is critical to accurate diagnosis and optimum care. This allows physicians to see organs and structures not typically visible during endoscopy alone, such as the layers of the gastrointestinal tract wall, the liver, pancreas, lymph nodes and bile ducts.