Active Sound Examination (Percussion, Echo, Doppler)
The earliest "active" acoustic diagnostic technique was percussion
of the chest wall. A skilled clinician can use
this method to detect consolidation of the lungs, pleural effusion, and a few other
chest pathologies. Though based on transmission and reflection of sound, percussion
is purely qualitative and is unable to accurately localize pathologic changes. Modern
ultrasound improves on percussion by using shorter-wavelength sound waves and quantitative
detection of their reflections (echoes). The
Figure 30-25
Wave size and resolution. Wave reflection depends on
the relative size of waves and objects. For small objects, small waves are needed
for good reflection and resolution. A, The object
size is large relative to the wave size, which results in good spatial resolution.
B, The object size is small relative to the wavelength,
which leads to poor resolution of the objects. This principle applies to light waves
also. Hence, electron microscopes can resolve smaller objects than light microscopes
can because the wavelengths used are smaller. Imagine running an unsharpened pencil
over a fine (1-mm grid) or coarse (1-cm grid) screen. The pencil sticks in the larger
holes while passing over the smaller ones. Sharpening the pencil to a point improves
the resolution.
resolution of an examination is limited by the wavelength of the sound used ( Fig.
30-25
). Using ultrasound at frequencies in the megahertz (106
cycles/sec) range allows resolution of much smaller objects. The wavelength of 1.0-MHz
sound waves in solid tissue is about 1.5 mm, whereas the wavelength of a 256-Hz tone
(middle C) in tissue is 5.7 m.
With the use of esophageal transducers, echocardiography has become
a popular intraoperative monitoring technique.[16]
[17]
Sound waves in the 2- to 10-MHz range are
transmitted
toward the heart in short bursts or pulses. After each pulse, the transducer passively
listens to the reflected echoes from various tissues. The ability to place the transducer
in the esophagus is advantageous because sound does not then have to pass through
air spaces or bone on its way to and from the heart. The speed of sound through
the heart and surrounding soft tissues is a nearly constant 1540 m/sec. Thus, the
elapsed time between transmission of the pulse and receipt of the echo provides the
distance to the reflecting structure. The sound beam from the transducer is projected
in a narrow "searchlight" pattern, so the exact direction of reflecting structures
is also known.
The Doppler effect is used in echocardiography to determine the
presence and degree of valvular regurgitation by converting the Doppler shift of
sound waves reflected from erythrocytes into a color display (see Appendix
7
; also see Chapter 33
).
Cardiac output has been estimated from descending thoracic aortic blood velocity
by using a Doppler technique. These devices estimate the blood flow in the descending
aorta and ignore flow to the head and arms. They calibrate descending aortic flow
to cardiac output by assuming a constant proportional relationship between the two
flows.
