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Appendix C In Situ, Derated, and
Water Value Intensities
All intensity parameters are measured in water. Since water absorbs very little acoustic energy, these water
measurements represent a worst case value. Biological tissue does absorb acoustic energy. The true value of the
intensity at any point depends on the amount and type of tissue and the frequency of the ultra-sound that passes
through the tissue. The intensity value in the tissue, In Situ, has been estimated by using the following formula:
In Situ = Water[e
-0:23alf
]
where: In Situ = In Situ Intensity Value
Water = Water Value Intensity
e = 2.7183
a = Attenuation Factor
Tissue = a (dB/cm-MHz)
Amniotic Fluid = 0.006
Brain = 0.53
Heart = 0.66
Kidney = 0.79
Liver = 0.43
Muscle = 0.55
l = Skin line to measurement depth (cm)
f = Center frequency of the transducer/system/mode combination (MHz)
Since the ultrasonic path during an examination is likely to pass through varying lengths and types of tissue, it is
difcult to estimate the true In Situ intensity. An attenuation factor of 0.3 is used for general reporting purposes;
therefore, the In Situ value which is commonly reported uses the formula:
In Situ derated = Water[e
-0.069lf
]
Since this value is not the true In Situ intensity, the term “derated” is used.
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