Attenuation

Published on 06/02/2015 by admin

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Last modified 22/04/2025

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3 Attenuation

Attenuation is a decrease in wave amplitude as it travels through a medium. The attenuation of ultrasound in soft tissue is about 0.8 dB/(MHz-cm), indicating that the extent of attenuation depends on the distance traveled and the frequency of insonation. The units of the attenuation coefficient directly show the greater attenuation of high-frequency ultrasound beams. In soft tissue, 80% or more of the total attenuation is caused by absorption of the ultrasound wave, thereby generating heat.

Time gain compensation (TGC) adjusts for attenuation of an ultrasound beam as a function of depth. When TGC is properly adjusted, images of similar reflectors appear the same regardless of depth.

An acoustic shadow is said to exist when a localized object reflects or attenuates sound to impede transmission. Bone is a strong absorber of ultrasound waves. Therefore, shadowing occurs deep to bony structures (“bone shadow”).

When a nonattenuating fluid (e.g., blood or injected local anesthetic) lies within an attenuating sound field (e.g., soft tissue), enhancement of echoes deep to the fluid occurs. This phenomenon, originally described as posterior acoustic enhancement (also called increased through-transmission), is due to lack of absorption of the sound waves by the fluid.¹ This attenuation artifact is a potential source of problems, especially during regional blocks where nerves are situated close to blood vessels.

Clinical Pearls

• In general, the highest frequency capable of adequate penetration to the depth of interest should be used for imaging.

• Decibels (dB) are a relative logarithmic measure of sound wave intensity.

FIGURE 3-1 Acoustic shadowing by bone. In this sonogram from the forearm, the acoustic shadowing by the ulna is evident. The bright cortical line of the surface of the bone is followed by extinction of the sound wave below.

1 Filly RA, Sommer FG, Minton MJ. Characterization of biological fluids by ultrasound and computed tomography. Radiology. 1980;134:167–171.

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Atlas of Ultrasound-Guided Regional Anesthesia 2e