Attenuation (ultrasound)

The amplitude and intensity of ultrasound waves decrease as they travel through tissue, a phenomenon known as attenuation. Given a fixed propagation distance, attenuation affects high frequency ultrasound waves to a greater degree than lower frequency waves. This dictates the use of lower frequency transducers for deeper areas of interest, albeit at the expense of resolution.

Physics

Attenuation is the result of several features of sound wave interaction with tissue and tissue boundaries, including ¹;

• absorption
• scatter
• reflection
• divergence
• diffraction
• interference

Conversion of transmitted energy to another form of energy such as heat (absorption) is the primary means by which attenuation of ultrasound occurs in biologic tissue, with scatter comprising the other significant contributing factor. The intrinsic propensity of a medium to attenuate sound waves at a given frequency may be represented by its attenuation coefficient (represented by the greek letter alpha, and measured in dB/[mHz x cm]). The following are examples of the attenuation coefficients and properties of some commonly encountered tissues ⁵:

• highest attenuation coefficients
  • aerated lung (α > 34.0) presents a virtually impermeable barrier to ultrasound, as does cortical bone (α ~ 20.0)
    • loss of lung aeration will alter the degree to which it attenuates ultrasound waves
• lowest attenuation coefficients
  • water and blood attenuate ultrasound waves to a hardly appreciable degree (α ~ 0.18)

Other soft tissues, such as organs or skeletal muscle, have attenuation coefficients which lie between the aforementioned extremes.