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a Chapter 8 Biological Safety of Diagnostic Sonography 97
Acoustic Characteristics
Generation of heat in the insonated tissue depends
essentially on the power and intensity of the ultrasound.
Several acoustic features are involved in this
process including the following:
a. Acoustic power: The most relevant power parameter
for thermal bioeffect is the spatial-peak temporal-average
intensity (I SPTA ) which, as discussed
in Chap. 2, is the highest time-averaged acoustic
intensity at any point in the field. The output intensity
varies with the application-specific default
settings such as fetal, cardiac, or peripheral vascular
investigations. The operator can supersede such
regulations in choosing the mode. Most devices
provide a control for altering the power output.
Caution must be exercised in increasing the power
indiscriminately as most obstetrical ultrasound examinations
can be performed efficiently with the
acoustic power set well below the regulatory limits.
These issues will be discussed again.
b. Focus: The I SPTA increases with focusing, which
concentrates the power in a small area, causing
higher intensities and increasing the potential for
heating. Most diagnostic instruments employ a focused
beam in order to enhance lateral resolution
and directionality.
c. Scanning vs stationary beam: The I SPTA intensity is
affected by whether the ultrasound device utilizes
a scanning or a stationary beam. In the scanning
ultrasound modes, which include B-mode and color
flow Doppler imaging, the moving beam temporally
distributes the acoustic energy over a wide
volume of tissue in the scanned area and thus
minimizes the risk of tissue heating. In the stationary
ultrasound modes, which include spectral
Doppler and M-mode, the acoustic power is concentrated
linearly along the static ultrasound beam
axis, depositing acoustic energy in a significantly
smaller volume of tissue for the duration of exposure.
In this scenario, the highest elevation of temperature
is encountered along the beam axis between
the surface and proximal to the focal point,
and its location is near the surface if the focal
length is long and adjacent to the focal point if the
focal length is short.
d. Pulse repetition frequency (PRF): The higher the
PRF, the greater the temporal average intensity.
The PRF is under the user control, but can change
automatically interactively with other controls such
as the focal range. Increasing the focal range may
automatically elevate the PRF. In spectral pulsed
Doppler and color Doppler modes, the PRF is increased
to eliminate aliasing which can result in
increased I SPTA .
e. Pulse length (also known as the pulse duration or
burst length): This directly affects I SPTA . This is
relevant for using the pulsed Doppler mode, as a
larger Doppler sample volume will increase pulse
duration which will increase the intensity. The
pulse length is also greater in color flow Doppler
than in B-mode imaging.
f. Dwell time: This refers to the duration of ultrasound
exposure. The longer the dwell time, the
greater the thermal effect. During most fetal examinations
the operator moves the transducer
around, thereby reducing the duration of exposure
in a given location. In using unscanned modes
such as spectral Doppler interrogation of the middle
cerebral artery, one needs to reduce the scan
time deliberately because of the greater risk of
thermal effect in this specific instance.
g. Write zoom: In this function, where the image is
magnified by rescanning a smaller area of interest
in the image plane with more scan lines, the insonated
tissue is exposed to a higher concentration
of acoustic intensity. Moreover, using the write
zoom box at a greater depth will lead to an even
higher intensity as this requires a larger aperture
involving more transducer elements emitting more
power. This is especially applicable to the color
flow Doppler color box function where a narrower
and deeper color box will increase the intensity
and the risk of temperature elevation. This risk is
enhanced when color flow Doppler is used along
with spectral pulsed Doppler in the duplex mode.
h. Transducer frequency: Higher frequency ultrasound
is more avidly absorbed by tissues, increasing
the risk of heating. As higher frequency limits
the depth of penetration, heating is restricted to
the superficial tissues close to the transducer.
Moreover, suboptimal depth resolution in this case
may prompt the user to increase the power output,
which may contribute to tissue heating.
i. The nonlinearity of ultrasound propagation: In
ultrasound pulses, especially the high amplitude
waves, the compression in the wave propagates
faster than the rarefaction, resulting in distortion
in the waveform so that the peak of the wave follows
the trough very closely. With the compression
following the rarefaction very quickly, shock and
harmonic components of higher frequencies are
generated. This phenomenon is impeded in tissue
with higher attenuation such as bone and is facilitated
in tissue with lower attenuation such as amniotic
fluid or even neural tissue. The higher frequency
may lead to significant absorption of energy
and conversion to heat. Although such thermal
effects have not been demonstrated in the fetus,
the potential exists, especially when ultrasound
of higher amplitudes propagating through a