Introduction to Acoustics

provided simple metrics for the likelihood of such effects,
the thermal index (TI) and mechanical index (MI).
Similar standards have been addressed in other
countries by organizations and agencies including
Health Canada, the European Committee for Medical
Ultrasound Safety (ECMUS), the British Medical
Ultrasound Society (BMUS), the European Federation
of Societies for Ultrasound in Medicine and
Biology (EFSUMB), the Australasian Society for Ultrasound
in Medicine (ASUM), the Asian Federation for
Societies of Ultrasound in Medicine and Biology (AF-
SUMB), the Latin American Federation of Ultrasound in
Medicine and Biology (FLAUS), the Mediterranean and
African Society of Ultrasound (MASU), and the World
Federation for Ultrasound in Medicine and Biology
(WFUMB).
In spite of the confidence that ultrasound is absolutely
safe, all ultrasound agencies subscribe to the
policy of using ultrasound exposure levels that are as
low as reasonably achievable (ALARA) for diagnostic
ultrasound examinations.
In the presence of ultrasound contrast agents or gas in
the lungs or gut, there is a risk of damaging capillaries
and creating interstitial hemorrhaging in tissues when
high-MI examinations are performed.
However, there are some other considerations. In
addition to heating tissue and the compression/decompression
mechanical factors related to mechanical index,
an ultrasound wave exerts radiation pressure on tissue as
it passes through. The radiation pressure is proportional
to intensity. Radiation force is used in the measurement
of acoustic output. The body force is equal to the [power
entering a region minus the power transmitted through
References
21.1 G.E. Horn, J.M. Mynors: Recording the bowel
sounds, Med. Biol. Eng. 4(2), 205–208 (1966)
21.2 T. Tomomasa, A. Morikawa, R.H. Sandler,
H.A. Mansy, H. Koneko, T. Masahiko, P.E. Hyman,
Z. Itoh: Gastrointestinal sounds and migrating
motor complex in fasted humans, Am. J. Gastroenterol.
94(2), 374–381 (1999)
21.3 C. Leknius, B.J. Kenyon: Simple instrument for auscultation
of temporomandibular joint sounds, J.
Prosthet. Dent. 92(6), 604 (2004)
21.4 S.E. Widmalm, W.J. Williams, D. Djurdjanovic,
D.C. McKay: The frequency range of TMJ sounds,
J. Oral. Rehabil. 30(4), 335–346 (2003)
21.5 C. Oster, R.W. Katzberg, R.H. Tallents, T.W. Morris,
J. Bartholomew, T.L. Miller, K. Hayakawa: Characterization
of temporomandibular joint sounds.
Medical Acoustics References 895
and exiting the region] divided by the speed of sound.
At a pulsed Doppler focus in the examination of the fetus,
a temporal peak radiation pressure of 1700 Pa can
be generated at a frequency equal to the pulse repetition
frequency (PRF) of 5 kHz, which is well within
the hearing range. If the ultrasound beam pattern is
directed to intersect the ear, the radiation pressure oscillation
can be heard by the fetus and cause increased
fetal activity [21.94].
Of more serious concern is the increase in permeability
in cell membranes and other biologic barriers to
small and large molecules that can be induced by ultrasound
[21.95]. This may prove useful for the delivery of
therapeutic drugs through the skin (sonoporation) and
genes across membranes including the placental barrier
and the blood–brain barrier, but it might also allow the
inadvertent transfer of pathogens. The effect is enhanced
in the presence of gas and ultrasound contrast agents. It
is possible that such effects have escaped detection by
current monitoring methods.
The conceptual simplicity of the pulse-echo system
has allowed instrument engineers to devote all efforts
toward lowering the cost and improving the diagnostic
utility of these instruments. Unlike most other medical
imaging methods, ultrasound is low-cost, portable, realtime,
and considered safe in most applications.
Further Reading
Please find a complete compilation of citations from
Ultrasound in Medicine and Biology which list the thousands
of articles on medical acoustics written prior to
1987: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?
CMD=Display&DB=pubmed
A preliminary investigation with arthrographic correlation,
Oral Surg. Oral Med. Oral Pathol. 58(1),
10–16 (1984)
21.6 A. Peylan: Direct auscultation of the joints; Preliminary
clinical observations, Rheumatism 9(4), 77–81
(1953)
21.7 E.P. McCutcheon, R.F. Rushmer: Korotkoff sounds.
An experimental critique, Circ. Res. 20(2), 149–161
(1967)
21.8 J.E. Meisner, R.F. Rushmer: Production of sounds
in distensible tubes, Circ. Res. 12, 651–658 (1963)
21.9 S. Jarcho: The young stethoscopist (H. I. Bowditch,
1846),Am.J.Cardiol.13, 808–819(1964)
21.10 R.F. Rushmer, R.S. Bark, R.M. Ellis: Direct-writing
heart-sound recorder (The sonvelograph), Ama.
Am. J. Dis. Child 83(6), 733–739 (1952)
Part F 21