Chapter 2. Safety 2 - 35
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. A de-rating factor of 0.3 is
used for general reporting purposes; therefore, the In Situ value which is commonly reported
uses the formula:
In Situ (derated) = Water
)069.0( lf
e
Since this value is not the true in situ intensity, the term “de-rated” is used.
The maximum de-rated and maximum water values do not always occur under the same
operating conditions. Therefore, the reported maximum water and de-rated values may not be
related to the In Situ (de-rated) formula. For example, a multi-zone array transducer has the
greatest water value intensities in its deepest zone. The same transducer may have its largest de-
rated intensity in one of its shallowest focal zones.
▌ Acoustic Output and Measurement
The terms and symbols used in the acoustic output tables are defined in the following paragraphs.
ISPTA.3 The derated spatial-peak temporal-average intensity (milliwatts per square
centimeter).
ISPPA.3 The derated spartial-peak pulse-average intensity (watts per square
centimeter). The value of IPA.3 at the position of global maximum MI (IPA.3@MI)
may be reported instead of ISPPA.3 if the global maximum MI is reported.
MI The Mechanical Index. The value of MI at the position of ISPPA.3,
(MI@ISPPA.3) may be reported instead of MI (global maximum value) if ISPPA.3
is 190W/cm
2
.
The derated peak rarefactional pressure (megapascals) associated with the transmit pattern
giving rise to the reported Pr.3 MI value.
WO Ultrasonic power(milliwatts). For operating conditions giving rise to ISPTA.3, WO
is the total time-average power. For operating conditions subject to reporting
under ISPPA.3, WO is the Ultrasonic power associated with the transmit pattern
giving rise to the value reported under ISPPA.3.
Fc Center frequency (MHz). For MI and ISPPA.3, Fc is the Center frequency
associated with the transmit pattern giving rise to the global maximum value of
the respective parameter. For ISPTA.3, for combined modes involving beam types
of unequal Center frequency, Fc is defined as the overall ranges of center
frequencies of the respective transmit patterns.
ZSP The axial distance at which the reported parameter is measured (centimeters)
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