Endoscopic ultrasound (EUS) is a procedure which allows sonographic imaging of the luminal gastrointestinal (GI) tract together with surrounding structures and organs. The procedure has revolutionized the diagnosis and management of cancer by facilitating enhanced loco-regional imaging, ultrasound-guided fine-needle aspiration (FNA) and biopsy (FNB), placement of fiducials in tumor masses to guide delivery of radiation therapy, and injection of neurolytic agents to address cancer-associated pain. Rapidly evolving therapeutic EUS techniques and supporting devices now facilitate drainage of pancreatic pseudocysts and walled-off pancreas necrosis, bile duct and gallbladder drainage, and hemostasis for both variceal and nonvariceal hemorrhage.

Endoscopic ultrasound relies upon emission of high-frequency sound waves from the EUS probe. These sound waves travel freely through fluid and soft tissues; however, they are reflected back (as “echoes”) when they encounter a more solid (dense) structure. For example, the ultrasound waves will travel freely through bile (liquid) in the gallbladder, then echo back when the waves encounter a gallstone within the gallbladder. In this way, as ultrasound waves encounter structures of variable density within the body, echoes of varying strength result in an image of structures surrounding the ultrasound probe, with each exhibiting a unique density.

The echoendoscope is similar to a standard endoscope, but has oblique-viewing optics and an ultrasound transducer at its tip, together with a water inflatable balloon to enhance acoustic coupling. Dual purpose buttons on the endoscope handle allow for air insufflation and suctioning of the gastrointestinal lumen, as well as inflation and deflation of the balloon. The ultrasound transducer at the tip of the EUS scope emits high-frequency sound waves which travel through the tissue being evaluated.
A proportion of these transmitted waves are also reflected, refracted, scattered, and absorbed. The reflected waves are received by the same transducer as “echoes” and are processed to form an image of the area being evaluated. The whiteness or blackness of each point of the image on the gray scale depends on the amplitude of the echo, which in turn depends on the density of the tissue. EUS processors incorporate Doppler technology, which allows identification of blood vessels within the field of imaging and allows differentiation from other tubular structures such as the bile and pancreatic ducts. Doppler technology allows for identification of a safe pathway which avoids blood vessels for FNA, thereby minimizing the risk of bleeding as a procedural complication.

Obtaining an optimal image requires modulation of several factors. Sound waves travel best through water, whereas air creates noise and reverberations resulting in EUS image degradation. Optimal endosonographic images are therefore attained either by filling the gastrointestinal lumen with water, or by suctioning air out of the lumen and filling the echoendoscope balloon with water to improve acoustic coupling with the structure of interest. When imaging a wall lesion, the ultrasound beam should ideally be perpendicular to the lesion to avoid imaging artifacts that arise from tangential views. The area of interest should be in the focal zone of the ultrasound probe, usually about 1.5 cm from the probe.

There are two types of echoendoscopes: radial and linear. The radial echoendoscope scans in a plane perpendicular to the axis of the endoscope to produce an axial image similar to that of a computed tomography (CT) scan. The linear echoendoscope scans in a plane parallel to the axis of the endoscope shaft. This plane of imaging allows visualization of needles as they are advanced beyond the tip of the echoendoscope, thereby facilitating sampling of tissue or cystic fluid (FNA/FNB), and visualization of devices advanced through the echoendoscope channel (e.g., stents, coils, fiducials, neurolytic agents).

Ultrasound mini-probes that can be advanced through the accessory channel of standard endoscopes, either alone or over a guidewire, are available and may offer advantages in certain clinical scenarios. These include immediate assessment of subepithelial nodules incidentally noted during standard endoscopy, and assessment of lesions out of the reach of a standard echoendoscope (e.g., cecal lesions) and lesions where adequate alignment with a standard echoendoscope is difficult to achieve.