General principles of endoscopic ultrasonographic imaging

Abstract

Endoscopic ultrasonography (EUS) uses high-frequency acoustic waves within the gastrointestinal lumen to obtain detailed images of the gut wall and surrounding structures. Knowledge of basic ultrasound (US) physics is important for both acquisition of high-quality images and accurate interpretation. US waves are produced by transducers, which also act as receivers for reflected waves, or echoes. Images are formed by processing of the electrical signals resulting from reception of echoes by the transducer. Tiny pulses of US energy are emitted into tissue along a single plane by either a mechanically rotated transducer or a fixed array of transducers. A structure's depth is determined by the time it takes for the US pulse to return to the transducer. The intensity of a returning echo depends on the reflectivity of the target structure and the amount of energy loss, or attenuation, in tissue. Real time imaging is possible because of rapid renewal of images several times per second. Axial (or depth) resolution is determined by US pulse length; it is optimal at high US frequencies. However, tissue penetration decreases as frequency is increased, limiting the depth of imaging. Lateral resolution is a function of the width of each US wave or beam. Transducer size and frequency determine the shape and dimensions of the beam, which typically converges at a distance of 2 to 3 cm from the transducer and diverges thereafter. Lateral resolution is best in the narrowest portion, termed the focal zone . In general, optimal resolution occurs with use of the highest possible frequency that allows adequate penetration to the structures of interest. Two types of echoes are produced in tissue: scattering of US waves by tiny particles or irregular surfaces, and reflection at interfaces between layers of tissue with different acoustic properties. EUS imaging of the gut wall is used to illustrate several of the fundamental US principles detailed in this article. An overview of the basic types of EUS instruments and certain applications, such as Doppler or duplex scanning, is also provided.