1 - 34 HS70A Service Manual
▐In Situ, De-rated, and Water Value Intensities
All intensity parameters are measured in water. Since water does not absorb 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
ultrasound that passes through the tissue. The intensity value in the tissue, In Situ has been estimated
using the following formula:
In Situ = Water
where: In Situ = In Situ Intensity Value
Water = Water Value Intensity
e = 2.7183
a = Attenuation Factor
Tissue a(dB/cm-MHz)
Brain .53
Heart .66
Kidney .79
Liver .43
Muscle .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 difficult to estimate the true In Situ intensity. An attenuation factor of 0.3 is used for general
reporting purposes. Therefore, the In Situvalue which is commonly reported uses the formula:
In Situ (derated) = Water
Since this value is not the true in situ intensity, the term “de-rated” is used.
The maximum derated and the maximum water values do not always occur at the same operating
condition. Therefore, the reported maximum water and derated values may not be related to the In Situ
(derated) formula. For example, a multi-zone array transducer that has maximum water value intensities
in its deepest zone: The same transducer may have its largest derated intensity in one of its shallowest
focal zones.
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