Chapter 2. Safety 2 – 31
The maximum derated and maximum water values do not always occur under the same
operating conditions. Therefore, the reported maximum water and derated values may not be
related to the In Situ (derated) formula. For example, a multi-zone array transducer has the
greatest water value intensities at its deepest zone. The same transducer may have its largest
derated intensity at one of its shallowest focal zones.
2.5.1.14. Terms and Symbols Related to Acoustic Output and Measurement
The terms and symbols used in the acoustic output tables are defined in the following
paragraphs.
A
aprt
-12dB OUTPUT BEAM AREA, ultrasonic beam area induced by -12dB
output beam size (unit: cm
2
)
d
eq
at max. I
pi
EQUIVALENT BEAM DIAMETER, the acoustic beam's diameter at the
location where the PULSE-INTENSITY INTEGRAL is maximal,
expressed as an equivalent beam area (unit: cm)
d
eq(
z
b)
EQUIVALENT BEAM DIAMETER, the acoustic beam's diameter at Zb
location, expressed as an equivalent beam area (unit: cm
2
)
Dim of A
aprt
-12dB OUTPUT BEAM DIMENSIONS, the dimensions of an ultrasound
beam (whose pulse beam width is -12dB) from a specific direction that is
perpendicular to the transducer output plane and the beam alignment
axis (unit: cm)
f
awf
ACOUSTIC WORKING FREQUENCY, the arithmetic average of f1 and
f2 that are farthest from each other among the frequencies of the
pressure spectrum of the acoustic signal whose amplitudes are lower
than the peak amplitude, i.e. maximum amplitude, by 3dB (unit: MHz)
Focal Length The focal length in a direction parallel to the beam alignment axis in the
defined operational state of the ultrasound system (unit: cm)
I
pa,α
at max. MI
The average attenuated pulse strength at the location where Mechanical
Index (MI) is maximal (unit: W/cm
2
)
I
ta, α
(z)
Attenuated average strength over time at a specific focal length (z) (unit:
mW/cm
2
)
MI MECHANICAL INDEX, a variable representing potential cavitations within
the human body (unit: N/A)
P OUTPUT POWER, the average power over time of an ultrasonic
transducer's emissions into a free field through a specified medium such
as water (unit: mW)
P
α
(z) ATTENUATED OUTPUT POWER, the power of ultrasonic output
calculated at a specific distance from the transducer after attenuation
occurs (unit: mW)
P
r,α
ATTENUATED PEAK-RAREFACTIONAL ACOUSTIC PRESSURE, the
peak rarefactional acoustic pressure calculated at a specific distance
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