FujiFilm SonoSite X-Porte User Manual page 273

The acoustic output for this ultrasound system has been measured and calculated in accordance with
"Acoustic Output Measurement Standard for Diagnostic Ultrasound Equipment" (NEMA UD2-2004), and
"Standard for Real-Time Display of Thermal and Mechanical Acoustic Output Indices on Diagnostic
Ultrasound Equipment" (NEMA UDe3-2004).
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:
-(0.23alf )
In Situ= Water [e
where:
In Situ = In Situ intensity value
Water = Water intensity value
e = 2.7183
a = attenuation factor (dB/cm MHz)
Attenuation factor (a) for various tissue types are given below:
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 n 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
Since this value is not the true In Situ intensity, the term "derated" is used to qualify it.
Acoustic output
] where:
-(0.069lf )
]
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