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Published online by Cambridge University Press: 05 July 2011
Regarding the properties of ultrasound:
Ultrasound propagates through tissue as a longitudinal wave
The velocity of the ultrasound wave is equal to the velocity of the particles of the tissue through which it propagates
The wavelength of ultrasound in tissue in the range of frequencies used in diagnostic imaging is 1–5 mm
An ultrasound wave undergoes reflection and refraction but not Rayleigh scattering
Attenuation of the ultrasound wave by 10 dB corresponds to a tenfold decrease in ultrasound intensity
Ultrasound velocity:
Is equal to the ultrasound frequency divided by the wavelength
Increases with frequency in a given medium
Is proportional to the square root of the material density
Is higher in compressible materials
Is lower in tissues with a higher fat or water content
Which of the following are true about acoustic impedance?
It is measured in kg m−2
The acoustic impedance of fat is lower than most other tissues
The acoustic impedance of air is almost zero
It is independent of temperature
The intensity of ultrasound is directly proportional to the acoustic impedance and to the wave amplitude
Concerning the reflection of ultrasound waves:
Specular reflection occurs when the size of the reflector is much larger than the ultrasound wavelength
The angle of specular reflection depends on the differences in acoustic impedance of tissues forming the boundary and is described by Snell's law
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