o_1977r8vv9vk1ts2ms0kd8pksa.pdf

102 D. Maulik
liquid, the oscillatory motion generates a steady eddying
flow, often of high velocity; this phenomenon
is known as microstreaming. Intracellular microstreaming
can also occur from a vibrating cell membrane
close to an asymmetrically oscillating cavity.
That stable cavitation can induce bioeffects under experimental
conditions has been demonstrated by various
investigators [39, 40]. However this type of cavitation
has not been shown to be of any significance
in bioeffects considerations.
Cavitational phenomenon in a medium related to
ultrasound exposure is controlled by many factors including
acoustic characteristics of field, ambient factors,
and cavitational potential of the medium. These
are elaborated below.
Acoustic Characteristics for Cavitation
The critical acoustic factors responsible for producing
inertial collapse of a resonating bubble are the maximum
rarefaction and compressional pressures. The
greater the rarefactional or negative pressure, the
more the bubble expands preceding collapse. The
greater the compressional or positive pressure, the
more intense is the inertial pressure of the collapse.
Both factors determine the intensity of the bubble
collapse and the consequent severe local disruption.
However, positive or negative pressure spikes of very
short duration (< 0.01 ls) are not of significance.
With pulsed ultrasound, the pulse length, duty cycle
and frequency affect cavitation. Shorter pulses and
lower pulse repetition frequency increase the cavitational
intensity threshold. Indeed, it has been suggested
that cavitation may probably be prevented if
the pulse duration is sufficiently reduced [41], which
has led to the general assumption that with medical
diagnostic pulsed echo ultrasound, the pulse duration
is too short to produce any cavitational activity. However,
it has been demonstrated that microsecondlength
pulses used in diagnostic pulsed-echo equipment
may cause inertial cavitation if gas nuclei of
suitable size are present in the medium and if the
acoustic parameters of the ultrasound are appropriate.
Moreover, the intensity threshold for producing
inertial cavitation has been shown to be 1±10 W/cm 2
for microsecond-length pulses [42]. Temporal maximum
intensity far exceeding this level can occur in
diagnostic ultrasound imaging. The risk is potentially
greater with some pulsed Doppler systems because of
the higher pulse repetition frequency and intensity.
Ambient Pressure and Temperature
Ambient pressure and cavitational activity are inversely
related, so an increase in the former increases
the threshold intensities for cavitation. Conversely,
higher temperature facilitates cavitation by decreasing
the solubility of gas in the medium which will lead to
microbubbles that serve as nuclei for cavitation to occur.
Tissue Characteristics for Cavitation
Prerequisites for cavitational activity are the quantity
and size of the gas nuclei present in the medium. As
mentioned earlier, even microsecond-length pulses
have the potential to induce inertial cavitation, depending
on the presence of micronuclei of sufficient
size. Unfortunately, gaseous micronuclei are not easily
detected. Although it has been disputed in the past,
there is ample evidence that strongly suggests the
presence of such nuclei in mammalian tissues. Such
evidence includes studies on decompression syndrome
in humans, experiments with lithotripter in
dogs [43], and the observation in mice that application
of hydrostatic pressure increased the threshold
for sonar-related tissue damage [44]. The presence of
such nuclei and insonation-related cavitation has
been demonstrated in mammals by Lee and Frizzell
[45] who observed hydrostatic pressure and temperature-dependent
neonatal mouse hind limbparalysis
due to ultrasound exposure. Aerated lung tissue with
its blood ± gas interface is particularly susceptible to
the presence of gas nuclei. This is further discussed
below.
Biological Effects of Inertial Cavitation
A collapsing cavity can result in cell lysis, dissociation
of water vapor, and generation of free radicals.
The mechanism by which inertial cavitation causes
cell destruction involves generation of intense local
heat and pressure and the generation of shear forces
by the bubble implosion [14, 46]. Furthermore, it has
recently been suggested that such implosions may occur
within the cell; such an event may not lyse the
cells but may give rise to free radicals. Continuouswave
high-intensity ultrasound has been shown to
produce free radicals in aqueous biological medium
by inertial cavitation [47]. The free radicals may affect
macromolecules. Thymine base alteration, chromosomal
agglomeration, and mutation have been observed
in insonated but intact cells. Irreversible deterioration
of the enzyme A-chymotrypsin has been reported
following production of radicals from insonation-induced
cavitation [48]. Similarly, other toxic
products such as hydrogen peroxide may induce adverse
effects in a biological system. It should be emphasized
that these phenomena have been noted
mostly during in vitro experiments and never in relation
to any human exposure.