Lapis ISSN 2398-2969

Ultrasound: physics

Contributor(s): Richard Saunders, Fraser McConnell, Anna Meredith

Introduction

  • Ultrasound is high frequency sound (1.5-15+ MHz used for veterinary ultrasonography). 
  • Pulse echo principle is used whereby a pulse of sound is emitted and an echo is returned after interacting with tissue. 
  • The expected course of the ultrasound beam can be measured and plotted by assuming a constant velocity of sound. 
  • There are many artefacts in ultrasonography and it is important to evaluate every organ in at least two planes to ensure any lesion is genuine.

Tissue characteristics 

  • The appearance of an organ depends mainly upon the degree of reflection, attenuation and scattering of the sound beam. 
  • A large difference between the acoustic impedance (acoustic impedance = velocity x density) of two tissues results in a large degree of reflection of the sound. 
  • Speed of sound depends on the compressibility known as 'elastic modulus' (k) and mass density of the material (p), and in most soft tissue is similar ~1540 m/s (c = square root of k/p). 
  • The wavelength of the sound beam in tissue is between 0.1 and 0.5 mm. 
  • If there is a large difference in density or velocity of sound, eg as between soft tissue and gas or bone and soft tissue, most of the sound is reflected resulting in a highly echogenic interface with no through transmission of the sound. 
  • This is why ultrasonography of the lung is unrewarding unless there is collapse or lung mass touching the chest wall. 
  • The fine speckled appearance of the parenchymal organs is due to scattering of the sound beam by small uneven structures. 
  • The individual dots seen on the screen do not represent an actual structure but are the sum of several small echoes. 
  • In general each organ has its own echogenicity and echotexture but the appearance will depend upon the machine settings and transducer frequency so it is important to compare organs with each other to be certain abnormalities are genuine. 
  • Order of increasing echogenicity of body tissues (taken from Veterinary Diagnostic Ultrasoundby Nyland and Mattoon): 
    • Bile, urine, ie fluid. 
    • Renal medulla. 
    • Muscle. 
    • Renal cortex. 
    • Liver. 
    • Storage fat. 
    • Spleen. 
    • Prostate. 
    • Renal sinus. 
    • Structural fat, vessel walls. 
    • Bone, gas, organ boundaries.

Interactions 

  • The most important ultrasound interactions are: 
    • Reflection. 
    • Refraction. 
    • Scattering. 
    • Absorption.

Reflection 

  • The amount of energy reflected and transmitted at an interface is dependent on the acoustic impedance (z) of the 2 tissues. 
  • Acoustic impedance (z) = density (p) x velocity of sound in tissue. 
  • Amount reflected = (z1-z2)/(z1+z2) squared. 
  • A large difference in acoustic impedance will result in most of the sound beam being reflected (as at soft tissue/air interfaces where 99% is reflected).

Specular reflection 

  • For the echo to be returned the interface must be larger than the wavelength of the beam and must be smooth compared to the wavelength. 
  • The sound beam must meet the reflector perpendicular for the echoes to be detected by the transducer.

Refraction 

  • The ultrasound may be refracted if there is a difference in speed of transmission between 2 tissues.
  • This is most commonly seen with cystic structures where there is refraction of the beam at the edge of the cyst resulting in a linear streak of reduced signal distal to the cyst.

Scatter 

  • Predominant interaction in most tissues. 
  • Dependent on frequency (f) (proportional to f squared). 
  • Occurs when the size of the scatterer is similar to the wavelength. 
  • Sound is scattered in all directions. 
  • Scatter produces the fine speckled appearance of parenchymal organs. 
  • The speckles do not represent actual structures but scattered echoes.

Absorption 

  • Most of the sound energy is absorbed by the tissues.
  • Increases with frequency and limits the maximum frequency that can be used for imaging.

Attenuation 

  • As the beam passes through tissue it is reduced in intensity due to the above interactive processes.
  • The degree of attenuation is tissue specific and is proportional to frequency.

Equipment

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Type of scan

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Safety

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Further Reading

Publications

Refereed papers

Other sources of information

  • Nyland T G & Mattoon J S (1995) Veterinary Diagnostic Ultrasound. W B Saunders. ISBN: 0721627455.


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