Introduction to Acoustics

892 Part F Biological and Medical Acoustics
Part F 21.11
A water path is used to couple the reflector to the
skin nearest the affected kidney so that the target focus
can be positioned on the kidney stone. By firing
the spark gap in the water at the source focus, pressures
of +100 MPa followed by −10 MPa are achieved
at the target focus. Treatment of a kidney stone may
require 3000 shocks delivered over an hour. The object
of the treatment is to break a large stone (≥ 1cm
in diameter) into smaller pieces (≤ 2 mm diameter) that
21.9 Thrombolysis
A new therapeutic application of ultrasound is the
destruction of blood clots occluding vessels. Two
mechanical methods may contribute to clot destruction
[21.83–85]: the formation of microbubbles in
the clot (cavitation) and differential radiation force
on the clot squeezing and expanding the clot like
a sponge to take up thrombolytic drugs. There are
21.10 Lower-Frequency Therapies
Lower-frequency ultrasound is used for tissue disruption
and cleaning purposes. To remove an opacified lens
(cataract) from the eye through a small incision, a needle
oscillating at 25–60 kHz is used to emulsify the
lens (phacoemulsification), which is then removed by
aspiration (suction).
21.11 Ultrasound Safety
Undoubtedly, ultrasound is safe. In over a half century
of widespread use of diagnostic ultrasound, there are
no currently accepted reports of harm to patients or
examiners from the use of diagnostic ultrasound. However,
in 1950, unamplified X-ray fluoroscopic imaging
was considered so safe that it was used for shoe fitting
in nearly all shoe stores, and for the entertainment of
children waiting to be served. In 1950, smoking was
considered to be so beneficial to health that nearly all
doctors smoked. Although there are reports that ultrasound
examination in pregnancy is associated with
increased birth complications in retrospective studies
where ultrasound was applied to high-risk pregnancies,
a causative effect is unlikely. Fewer complications were
the ureter can carry to the bladder. The mechanism of
stone destruction includes erosion, spallation, fatigue,
shear and circumferential compression associated with
the primary shock wave or resulting cavitation. Erosion
is caused by cavitation. Spallation is the ejection of the
distal portion of the stone, resulting from a decompression
wave reflected from the distal portion of the stone.
Unfortunately, lithotripsy is often associated with injury
to the kidney parenchyma [21.82].
research reports of successful thrombolysis in coronary
arteries [21.86], intracranial arteries [21.87, 88]
and vascular access shunts for dialysis. The ultrasound
intensities required for thrombolysis are similar
to those used in diagnostic ultrasound. The mechanisms
include streaming and disaggregation of the
clot [21.89].
Dental scaling is performed with a similar instrument
operating at frequencies of 18–30 kHz and applied
power of 30 W. Cavitation is an important mechanism in
the cleaning process. Chemical additives such as toothpaste
can suppress cavitation and degrade the cleaning
efficiency.
found in pregnancies randomized to ultrasound examination
compared to those with conventional care [21.90]
because of the increased knowledge of physicians about
the expected date of delivery. In one discussion of
the safety of Doppler ultrasound examination of the
ophthalmic artery through the eye, an experienced investigator
stated “If the patient doesn’t see flashes
of light, I figure that the exam is safe”. Still today,
diagnostic ultrasound is considered so safe that, although
there are occasional disclaimers such as “to be
used only under the supervision of a physician”, ultrasound
examinations are performed for educational
purposes on students, demonstrated on hired actors at
trade shows, available for home use, and advertised