Equipment, Principles, Technique, and Complications

Successful execution of EUS requires both specific equipment and special technique. A basic understanding of ultrasound imaging is necessary. Image interpretation requires a thorough knowledge of anatomical relationships of intra-abdominal vessels and organs [ 1, 11- 16]. Also, the ability to use a large diameter side-viewing endoscope is very helpful.

Equipment and Principles

Two methods of ultrasound scanning, electrical and mechanical, have been applied to intraluminal sonography. The linear sector echoendoscope electronically switches an array of piezoelectric transducers to produce a linear scan image that is parallel to the endoscope shaft. This image orientation has allowed the development of biopsy techniques. The biopsy needle can be directed to the target because it is in line with the scanning plane.

The mechanical sector echoendoscope uses a motor to rotate a single piezoelectric transducer to produce a 360 degree circular scan that is perpendicular to the endoscope shaft. Rotary electrical contacts transmit the transducer's signal to the processor. This type of scan facilitates orientation of the probe in the lumen of the GI tract. The mechanical sector echoendoscope has been the most widely used method, because rapid orientation allows faster, more efficient scanning. The Olympus (Olympus Corporation, Woodbury, NY) GF-UM systems are considered the most reliable echoendoscopes for initial scanning; however, biopsies with this method are more difficult because the needle is seen only as a single point where it crosses the ultrasound image plane that is perpendicular to the scope.

Echoendoscopes have a side or oblique angle of view, except for the echocolonoscope, which is forward viewing. The echocolonoscope has an accessory channel with optical components, which results in a wedge in the EUS image and only a 320-degree, clear sonographic image. The echoendoscope can be passed into the distal duodenum, and the cecum can be reached with the echo colonoscope. Current EUS instruments use high sound wave frequencies of 7.5 and 12 MHz, as compared with 3.5 MHz for transcutaneous ultrasonography (US) and 5 MHz for blind rectal and esophageal probes.

During EUS scanning, the transducer is guided through the GI lumen and placed directly adjacent to the area of interest. Bones, adipose tissue, and air-filled structures, which limit the sound wave imaging clarity of extracorporeal sonography, are avoided. Close proximity of the ultrasonic source to the organs imaged permits the use of high frequency, high resolution sound waves that have too short a penetration depth to be used in transcutaneous sonography. As with all sonography, the sound wave imaging plane can be oriented at any angle to bring the lesion into optimal focus, simply by turning the probe. The operator is not limited to parallel sections, usually 1 cm apart, as with computed tomography (CT).