Ultrasound interactions and attenuation

Chapter 8 Ultrasound interactions and attenuation



CONTENTS


Attenuation 45


LEARNING OBJECTIVES



1. Explain what is meant by the term attenuation.

2. Give the factors which influence attenuation.

3. Explain each of the interactions which are responsible for attenuation.

4. Describe how each of the above interactions affects the ultrasound image.


ATTENUATION


As the ultrasound beam travels through the body it loses energy. The intensity and amplitude of the sound wave decreases, and this process is known as attenuation.


The amount of attenuation that occurs will depend on the type of tissue the sound wave is traveling through. Where the molecules of the tissue are densely packed (such as bone), attenuation will be much greater than in less densely packed tissue (such as fat). Different tissues have different attenuation coefficients depending on the amount of attenuation occurring in the beam of sound.


Attenuation depends on the frequency of the sound. The higher the frequency, the greater the amount of attenuation that will occur in any given tissue.


Attenuation will occur not only in the beam of sound produced by the transducer as it propagates through tissue, but also in the returning echoes as they travel back to the transducer.


It therefore follows that returning echoes from deep within the patient will be of a much lower intensity than the initial beam of sound passing into the patient. It is for this reason that imaging equipment has a time-gain compensation (TGC) or depth-gain compensation (DGC) control, to allow for greater amplification of the weaker echoes returning from deeper within the body.


A variety of processes cause attenuation, but the five main processes to be considered are:


1. absorption

2. reflection

3. scattering

4. refraction

5. divergence.


Absorption


This is the main factor causing attenuation of the ultrasound beam. The higher the frequency of the sound wave, the greater the amount of absorption that will occur.


Energy is transferred from the sound wave into the medium through which it is traveling.


The rate of absorption depends on the absorption coefficient of the material through which it is traveling. Bone will have a much higher absorption coefficient than soft tissue. The absorption coefficient is proportional to the frequency. As an example, the absorption coefficient for a 7 MHz frequency wave is twice that of a 3.5 MHz frequency wave, e.g. a 3.5 MHz wave traveling through 1 cm of liver will have its intensity reduced by 50%, whereas a 7 MHz wave traveling through half this distance (0.5 cm of liver) will have its intensity reduced by the same amount.


Some absorption will be due to the energy lost from the sound wave in overcoming the opposition to its forward propagation. Other absorption will occur due to a transference of energy during the compressive part of the sound wave cycle.


The majority of the lost energy will cause a rise in temperature of the tissue through which the sound is traveling. This heating effect is a potential hazard when scanning biological tissue, although it can be used to advantage, for example, by physiotherapists when using ultrasound for treatment of soft-tissue injuries.



Reflection

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Mar 10, 2016 | Posted by in ULTRASONOGRAPHY | Comments Off on Ultrasound interactions and attenuation

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