Chapter 5: Safety 47
Safety
Acoustic Output Measurement
Since the initial use of diagnostic ultrasound, the possible human biological effects (bioeffects) from
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 (Bioeffects 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 current
information.
The acoustic output for this ultrasound system has been measured and calculated in accordance
with the “Acoustic Output Measurement Standard for Diagnostic Ultrasound Equipment” (NEMA
UD 2-1998, and the “Standard for Real-Time Display of Thermal and Mechanical Acoustic Output
Indices on Diagnostic Ultrasound Equipment” (AIUM and NEMA 1998).
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 a worst case value. Biological tissue does absorb acoustic energy.
The true value of the intensity at any point depends on the amount, type of tissue, and the frequency
of the ultrasound passing 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 intensity value
e = 2.7183
a = attenuation factor
tissue = a(dB/cm-MHz)
brain = 0.53
heart = 0.66
kidney = 0.79
liver = 0.43
muscle = 0.55
l = skinline to measurement depth in cm
f = center frequency of the transducer/system/mode combination in MHz
Since the ultrasonic path during the exam 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 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 to qualify it.
The maximum derated and the maximum water values do not always occur at the same operating
conditions; therefore, the reported maximum water and derated values may not be related by the
In Situ (derated) formula. For example: a multi-zone array transducer that has maximum water
value intensities in its deepest zone, but also has the smallest derating factor in that zone. The same
transducer may have its largest derated intensity in one of its shallowest focal zones.
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