Chapter 6: Safety 141
Safety
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 (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 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.
Tissue Models and Equipment Survey
Tissue models are necessary to estimate attenuation and acoustic exposure levels In Situ from
measurements of acoustic output made in water. Currently, available models may be limited in their
accuracy because of varying tissue paths during diagnostic ultrasound exposures and uncertainties
in the acoustic properties of soft tissues. No single tissue model is adequate for predicting exposures
in all situations from measurements made in water, and continued improvement and verification of
these models is necessary for making exposure assessments for specific exam types.
A homogeneous tissue model with attenuation coefficient of 0.3 dB/cm MHz throughout the beam
path is commonly used when estimating exposure levels. The model is conservative in that it
overestimates the In Situ acoustic exposure when the path between the transducer and site of
interest is composed entirely of soft tissue. When the path contains significant amounts of fluid, as
in many first and second-trimester pregnancies scanned transabdominally, this model may
underestimate the In Situ acoustic exposure. The amount of underestimation depends upon each
specific situation.
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