Applications of Endoscopic Ultrasonography

, and , Harbor-UCLA Medical Center, Torrance, California.

In This Article

Instrument and Technical Aspects

Two types of echoendoscopes are generally used in studying the gastrointestinal tract. One is a 360° radial scanner (eg, Olympus UM-20, UM-30 Echoendoscopes), the other, a sector scanner (eg, Olympus UC30P, Pentax FG-32UA Echoendoscopes).

The radial scanner produces a 360° view perpendicular to the shaft of the endoscope and has the option of scanning at 12 or 7.5 MHz (see Figure 1). The higher-frequency scanning (12 MHz) allows for better visualization of details at close range. The lower the frequency, the better the penetration of ultrasound waves; thus, at 7.5 MHz, the scanning range increases over 12 MHz. The radial echoendoscope is preferred by many endosonographers because it gives a 360° overview, similar to a computer tomogram, allowing complete visualization of the gastrointestinal tract and its adjacent structures.

Olympus GF-UM20 radial scanner with water-filled balloon on tip of endoscope.

In contrast, the sector scanner (eg, Pentax FG-32UA Echoendoscope, 7.5 and 5 MHz (see Figure 2), Olympus UC30P) scans parallel to the shaft of the scope and has a limited (100°) sector of scanning area. These echoendoscopes have color-flow and Doppler capabilities, which are useful in imaging vascular structures. FG-36UX and FG-38UX have recently replaced FG-32UA. Each of these instruments has larger biopsy channels (2.4 mm for FG-36UX and 3 mm for FG-38UX). Additionally, FG-36UX has an elevator for stent insertion.

Pentax FG-32UA sector scanner suitable for fine-needle aspiration (FNA).

The sector scanners are preferred for use in EUS-FNA because they allow direct visualization of the needle up to 5-6 cm parallel to the shaft of the endoscope. The radial scanner is difficult to use for FNA because the tip of the needle quickly leaves the scanning area, which is perpendicular to the tip of the endoscope. And, because it is essential to see the tip of the needle during FNA, most endosonographers have abandoned radial scanners in this setting.

The limited scanning range of ultrasound probes often necessitates a CT scan to evaluate lesions further from the gastrointestinal tract, such as distant metastases. The radial and sector echoendoscopes are equipped with forward-oblique or side-view optics, and operators should be familiar with using side-viewing endoscopes. A water-filled balloon over the transducer allows close contact of the transducer to suspected lesions in the gastrointestinal wall or adjacent structures. For the evaluation of lesions located in the stomach wall, filling the stomach with 200-500 mL of water often allows for detailed images of the gastric wall structures. Catheter-based ultrasound probes that employ high-frequency ultrasound (20 mHz) may be used through traditional videoendoscopes. These high-frequency ultrasound probes (HFUPs) are useful in evaluating small mucosal/submucosal lesions[3,4] in the luminal gastrointestinal tract.

Additionally, images may be obtained from within the lumen of pancreatic and bile ducts, thus enhancing the ability to detect early malignancy. Two recent studies demonstrated the enhanced diagnostic accuracy of these HFUPs over conventional EUS in the staging of early malignant lesions.[5,6] HFUPs are also useful in passing through narrow areas that are difficult to assess by endoscope and in performing endoscopic mucosal resection (EMR).