2 – 30 HS40 Service Manual
2.5.1.12. Acoustic Output and Measurement
Since the first usage of diagnostic ultrasound, the possible human biological effects (bioeffects)
of ultrasound exposure have been studied by various scientific and medical institutions. In
October 1987, the American Institute of Ultrasound in Medicine (AIUM) ratified a report prepared
by its Bioeffects Committee (Bioeffect Considerations for the Safety of Diagnostic Ultrasound, J
Ultrasound Med., Sept. 1988: Vol.7, No.9 Supplement), sometimes referred to as the Stowe
Report, which reviewed available data on possible effects of ultrasound exposure. Another
report, “Bioeffects and Safety of Diagnostic Ultrasound”, dated January 28, 1993, provides more
up to date information. In addition, periodically updated reports on biological effects, results, and
guidelines on safe usage have been published by groups such as the WFUMB (World
Federation of Ultrasound in Medicine and Biology), AIUM, and BMUS.
The Acoustic output for this system has been measured and calculated in accordance with the
Standard for Real-Time Display of Thermal and Mechanical Acoustic Output Indices On
Diagnostic Ultrasound Equipment (AIUM/NEMA, 2004) and Acoustic Output Measurement
Standard for Diagnostic Ultrasound Equipment (AIUM/NEMA, 2004).
2.5.1.13. In Situ, Derated, and Water Value Intensities
All intensity parameters are measured in water. Since water does not absorb acoustic energy,
these water measurements represent the largest possible value. Biological tissue absorbs
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 Situ value is commonly reported using this
formula:
In Situ (derated) = Water
Since this value is not the true in situ intensity, the term “derated” is used.
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