Chapter 3: Attenuation
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1. Attenuation
Attenuation can be defined as a decrease in wave amplitude (or
intensity) due to the mechanical wave interaction with the me-
dium. The word attenuation implies a decrease, so when referring
to attenuation, a negative sign is not used. Stating that a signal has
been attenuated by 10 dB is the same as saying the signal has been
reduced or decreased by 10 dB, or changed by -10 dB. It is incorrect
to state that the signal was attenuated by -10 dB, since this sentence
now has a double negative.
A comprehensive understanding of the mechanisms for attenua-
tion is crucial for understanding diagnostic ultrasound. They are
precisely what makes ultrasound work and not work. Without un-
derstanding attenuation effects, it is impossible to understand how
and why the controls of an ultrasound systemwork (Chapter 6), the
sources of artifacts (Chapter 8), the advanced topics of harmonic
imaging, contrast imaging (Chapter 10), and future ultrasound
developments.
We will divide the topic of attenuation into three subtopics, each
subtopic corresponding to a physical or mechanical interaction
with the medium. Specifically, we will subdivide attenuation into
the categories of:
• Absorption
• Reflection
• Refraction
KEY CONCEPT
2. Absorption
Absorption is a conversion of energy from the sound wave into heat
within the medium. The fact that energy is lost to heat within the
tissue should be intuitive from our description in Chapter 2 of how
a mechanical wave propagates. As the wave encounters the mol-
ecules (particles) of the medium, some of the energy is transferred.
This transfer of energy is responsible for the particle displacement
which results in compression and rarefaction. As with moving any
physical entity, energy is required. How much of the wave energy
is lost to heat through this interaction is dependent on the molecu-
lar interactions within the medium and the frequency of moving
(compressing) the medium.
2.1 Absorption and Viscosity
The molecular interaction is sometimes referred to as the viscosity
of the medium. A high viscositymedium implies that the molecular
attraction within the medium is high, requiring greater energy to
move the molecules within the medium. (Viscosity is formally dis-
cussed in Chapters 12 and 13.) Therefore,higher viscositymediums
result in greater energy losses to heat through absorption than lower
viscosity mediums.
2.2 Absorption and Frequency Dependence
If the frequency of the wave is increased, the amount of energy lost
to heat increases. This fact should be intuitive since an increased
frequency implies that the molecules are being “moved” more fre-
quently. The simple demonstration of this mechanical interaction
is rubbing your hands together. If you rub your hands together a
few times per second, not much heat is generated. If instead you
rub your hands together many times per second (higher frequency)
significantly more heat will be produced. Therefore, higher fre-
quency waves lose energy faster to heat through absorption than
lower frequency waves.
Attenuation
Chapter 3
