Technical Updates

Iqbal S. Sandhu, M.D.

EUS: What Happens If I Change the Frequency?

Endoscopic ultrasound (EUS) produces a gray-scale image of soft tissue within a region defined by a sound beam. To form an image, the transducer on the tip of the EUS endoscope is excited and it emits a sound pulse at an ultrasonic frequency at either 5, 7.5, 10 or 12 megahertz (MHz). The frequency chosen for transmission affects the depth at which tissue is visible as well as the resolvability of the structures imaged. This article will review how and why this occurs.

The emitted sound wave travels into the surrounding tissues, reflects off of structure interfaces and travels back to the transducer and EUS unit where it is processed to produce the image that we view. While traveling in tissue, the sound beam is progressively attenuated. Ultrasound (US) attenuation is caused by four phenomena: absorption, reflection, refraction and scattering. The image is produced by the sound that is reflected (echoed) back to the transducer. Absorbed sound is generally converted into tissue heating and does not contribute to the image signal. Refraction causes sound intensity to bend away from the expected tissue path and contributes only artifactually to the resulting image. Scattering is uniform sound reflection from small reflectors and produces the characteristic, internal "texture" of a structure in an image.

Figure 1

Figure 2

Reflection and refraction depend primarily on tissue properties and the speed and angle of the sound beam with respect to the tissue interfaces encountered. Thus on a typical EUS image (Figure 1) the interface between the serosal fat and muscularis propria is demonstrated with a bright intensity echo because of the large difference in acoustic impedance between the tissues and because of the perpendicular incidence of the sound beam onto their interface. Although the size of this reflected echo itself is not frequency dependent, its strength on an image does depend on how much of the sound beam is attenuated by tissue before it reaches the interface and while it is returning to the transducer for reception. Sound absorption and scattering are the major sources of this sound attenuation and thus affect the intensity of all echoes displayed on the EUS image. In the image in Figure 2, the lack of absorption and scattering of sound in the anechoic gall bladder results in the enhanced echo strength of the posterior wall as well as that of the scatterers lying behind the structure. Since both absorption and scattering are frequency dependent, the strength of any echo in an US image is also.


Ultrasound attenuation in tissue, when measured in decibels, is approximately directly proportional to frequency. Thus the attenuation of a 10 MHz sound beam is approximately twice that of a 5 MHz sound beam for the same tissue distance traveled. Thus a lower frequency EUS sound pulse will penetrate further into tissue and allow visualization of deeper interfaces. The EUS movie below shows the change in penetration of the US signal as the frequency of the EUS transducer is decreased.

Another frequency dependent image characteristic is the size or resolution of structures that are visible in an image. Because a higher frequency wave has a shorter wavelength, higher frequency EUS pulses can be made shorter. In addition the physics of focusing an EUS beam allows for the creation of narrower beams at higher frequencies. These two phenomena allow for creation of smaller viewing volumes at higher frequencies in tissue and lead to enhanced resolution for higher frequency EUS images.

In conclusion, the higher the frequency, the greater the resolution of adjacent structures, but the smaller the depth of imaging. For example, an EUS image at 10 MHz shows excellent resolution of the esophageal wall layers, but deeper structures cannot be discerned; imaging at 5 MHz demonstrates greater depth but the esophageal wall structures are not as clear.

References

Wolbarst AB. Physics of Radiology. Medical Physics Publishing, Madison, WI.

Editor:
John C. Deutsch, M.D.
Duluth, MN

Editorial Board:
Manoop S. Bhutani, M.D.
Galveston, TX
William R. Brugge, M.D.
Boston, MA
Peter R. McNally, D.O.
Denver, CO
Iqbal S. Sandhu, M.D.
Salt Lake City, UT
Thomas J. Savides, M.D.
San Diego, CA