Introduction to Acoustics

Medical
21. Medical
Acous
Acoustics
Medical acoustics can be subdivided into diagnostics
and therapy. Diagnostics are further separated
into auditory and ultrasonic methods, and both
employ low amplitudes. Therapy (excluding medical
advice) uses ultrasound for heating, cooking,
permeablizing, activating and fracturing tissues
and structures within the body, usually at
much higher amplitudes than in diagnostics. Because
ultrasound is a wave, linear wave physics
are generally applicable, but recently nonlinear
effects have become more important, even in
low-intensity diagnostic applications.
This document is designed to provide the
nonmedical acoustic scientist or engineer with
some insights into acoustic practices in medicine.
Auscultation with a stethoscope is the most basic
use of acoustics in medicine and is dependent
on the fields of incompressible (circulation) and
compressible (respiration) fluid mechanics and
frictional mechanics. Detailed discussions of
tribology, laminar and turbulent hemodynamics,
subsonic and supersonic compressional flow, and
surfactants and inflation dynamics are beyond
the scope of this document. However, some of
the basic concepts of auscultation are presented
as a starting point for the study of natural body
sounds. Ultrasonic engineers have dedicated over
half a century of effort to the development of
ultrasound beam patterns and beam scanning
methods, stretching the current technical and
economic limits of analog and digital electronics
and signal processing at each stage. The depth of
these efforts cannot be covered in these few pages.
However, the basic progression of progress in the
fields of transducers and signal processing will be
covered. The study of the interaction of ultrasound
with living tissues is complicated by complex
anatomic structures, the high density of scatterers,
and the constantly changing nature of the tissues
with ongoing life processes including cardiac
pulsations, the formation of edema and intrinsic
noise sources. A great deal of work remains to be
done on the ultrasonic characterization of tissues.
Finally, the effect of ultrasound
on tissues, both inadvertent and therapeutic will
be discussed.
Much of the medical acoustic literature
published since 1987 is searchable online, so this
document has included key words that will be
helpful in performing a search. However, much
of the important basic work was done before
1987. In an attempt to help the reader to access
that literature, Denis White and associates have
compiled a complete bibliography of the medical
ultrasound literature prior to 1987. Under Further
Reading in this chapter, the reader will find a link
to a complete compilation of 99 citations from
Ultrasound in Medicine and Biology which list the
thousands of articles on medical acoustics written
prior to 1987.
The academically based authors develop,
use and commercialize diagnostic ultrasonic
Doppler systems for the benefit of patients with
cardiovascular diseases. To translate ultrasonic
and acoustic innovation into widespread clinical
application requires as much knowledge about the
economics of medicine, the training and practices
of medical personnel, and the pathology and
prevalence of diseases as about the diffraction
patterns of ultrasound beams and signal-to-noise
ratio of an echo. Although a discussion of these
factors is beyond the scope of this chapter, a few
comments will help to provide perspective on the
likely future contribution of medical acoustics to
improved public health.
21.1 Introduction to Medical Acoustics.......... 841
21.2 Medical Diagnosis; Physical Examination 842
21.2.1 Auscultation – Listening
for Sounds................................ 842
21.2.2 Phonation and Auscultation ....... 847
21.2.3 Percussion................................ 847
21.3 Basic Physics of Ultrasound Propagation
in Tissue ............................................. 848
21.3.1 Reflection
of Normal-Angle-Incident
Ultrasound ............................... 850
839
Part F 21