Endoscopic Ultrasound




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Endoscopic ultrasound (EUS) is a new imaging modality where an ultrasound probe has been fitted to the tip of an endoscope. Traditional endoscopic examination was limited to visualization of the mucosa, EUS allows for depth analysis. Its applications span many fields in medicine such as gastroenterology, general surgery, oncology and thoracic surgery. The following will further elaborate on the many uses of EUS.

Transcutaneous ultrasound has gained widespread use in medicine as a safe, non-invasive method of visualizing internal organs and blood vessels. However its application has been limited by soundwave interference from bony structures and intestinal gas. Various blind probes (transanal, transvaginal and transesophageal) have recently been developed in an attempt to get closer to the organ studied (respectively, the prostate, the uterus and the heart). These probes are limited by their blind insertion. Endoscopic ultrasound was born from the need to combine endoscopic view and ultrasound imagery.

The sector scanning echoendoscope can be used to image many sites in the gastrointestinal tract: the esophagus, the stomach, the pancreas, the bile duct, the ampulla of Vater and the rectum. These will be discussed in sequential order.

EUS examination of the esophagus is mainly used for tumor staging. It is the only imaging modality which can determine accurately the depth of tumor invasion. This determines the treatment modality best suited for this tumor: surgery, chemotherapy, radiotherapy or a combination. It can also be used to study the nature of a submucosal lesion in the esophagus. EUS can differentiate between a cystic, vascular or solid lesion.

Gastric endosonography is also used to determine the nature of lesions located under the inner lining of the stomach. Again the EUS image can suggest whether a lesion is cystic, vascular or solid. It can also show from which layer of the stomach the tumor arises: the muscle layer, the fatty layer or is this mass located completely outside the stomach wall. No other imaging modality possesses enough resolution to reliably give this information.

Endoscopic ultrasound can also be used to examine the pancreas through the wall of the stomach and duodenum. It is the most sensitive test to detect small tumors of the pancreas. Surgical resection of pancreatic cancer while it is at an early stage is the only cure for this disease. EUS also has an important role in the staging of pancreatic cancer. It is the best tool to determine if the tumor has invaded surrounding blood vessels which would make it impossible to resect completely. This can avoid unnecessary operations. EUS has also been used to diagnose benign conditions of the pancreas such as acute and chronic pancreatitis.

EUS can also visualize the biliary tree and its opening in the bowel, called the ampulla of Vater. It has been shown to be as accurate as ERCP, the current gold standard, to diagnose stones in the gallbladder or bile duct, and it does not have the added risk of pancreatitis, allergy to the contrast agent or radiation exposure. It can also detect cancer of the bile duct and the ampulla as accurately as other imaging techniques. Here again, EUS appears to be the best modality for staging such cancers by determining the depth of tumor invasion and the presence of surrounding lymph nodes. New probes have been developed in this field which would be small enough to be inserted in the bile duct itself and allow even better ultrasonographic resolution.

Rectal EUS is mainly used to stage cancer of the rectum. The treatment of such a tumor is highly dependent on how deeply the cancer invades the rectal wall. This influences not only the need for chemotherapy or radiotherapy but also determines what type of surgery is indicated i.e. whether or not the anal sphincter can be preserved or if a more extensive surgery involving the creation of an ostomy must be performed.

The linear echoendoscope has similar applications to the sector scanning scope but has the added advantage of a biopsy channel where the needle can be seen in its long axis and not in cross-section.

While positioned in the esophagus, the EUS scope can visualize mediastianal lymph nodes, often involved in the spread of lung cancer. Their ultrasonographic appearance can be suggestive of malignancy but a firm diagnosis with all its implications cannot be made without microscopic analysis of tissue. Because of their location in the chest, sampling of those nodes until now involved a major operation. With EUS a fine needle can be positioned in the suspicious lymph nodes and a sample of cells can be obtained for analysis under the microscope. The determination of malignancy has major implications for the management of lung cancer. Positive lymph nodes in sites distant from the tumor means that this cancer would not be cured by a surgical resection and therefore that such a big operation is not indicated.

The linear array scope is also useful in the management of pancreatic cancer. Firstly, the fine needle aspiration technique (FNA) can be used to establish the diagnosis of a pancreatic malignancy. The pancreas is located very deep in the abdominal cavity, and even large tumors can be difficult to biopsy through the abdominal wall. The location of the scope in the duodenum or stomach allows closer proximity of the needle to the lesion and a safer biopsy path. EUS is also required to biopsy small tumors which can only be seen with the echoendoscope. Secondly, EUS-FNA can also be used in a manner similar as described above, to sample suspicious lymph nodes or fluid around the pancreas. Thirdly, EUS has been used to treat the pain associated with pancreatic cancer or chronic pancreatitis by using the needle to inject, under direct vision, sclerosing agents that destroy the nerves that transmit pain to that area.

EUS can also be used to treat pancreatic pseudocysts, a complication of pancreatitis. This collection of fluid outside the pancreatic gland can lead to pain and infection. Various methods to drain these have been described, one of which consists of endoscopic internal drainage through the wall of the stomach or duodenum. For this technique to be performed safely, there must be close proximity between the wall cyst and the intestinal tract, without intervening blood vessels as in assessed by EUS. Moreover, the linear echoendoscope can also allow the cyst to be punctured under direct vision rather than blindly as was traditionally done.

In conclusion, endoscopic ultrasonography is a rapidly growing field with widespread applications which will continue to expand. It is the single most important new technology development in gastroenterology since the advent of the fiberoptic endoscope.