Introduction to Acoustics

not as predicted, because the wave propagation speed in
the higher-pressure regions is faster than in the lowerpressure
regions. The waves convert from sinusoidal to
sawtooth, introducing a series of harmonic frequencies
into the wave. The change in frequency (wavelength)
affects the ultrasound beam pattern. The conversion of
ultrasound energy into harmonic frequencies also increases
the attenuation because the attenuation increases
with frequency.
A significant development was the use of tissuegenerated
harmonic signals for diagnostic imaging. The
utility of these harmonics was discovered by accident
when investigators were searching for better methods
of detecting ultrasound contrast agents. Ultrasound contrast
agents consist of microbubbles that have natural
21.4 Methods of Medical Ultrasound Examination
Over the half century of ultrasonic medical examination,
a variety of examination methods have been tried,
discarded, with some to be resurrected and honed for
use.
A passive method was evaluated to detect local
ultrasound emissions above background due to local
temperature elevations of cancerous tumors. The emissions
of about 10 −13 W/cm 2 (1 nW/m 2 ) are present in
all warm tissues and provide a minimum intensity for
the operation of active ultrasound systems. Transmission
ultrasound systems have been developed using an
ultrasound transmitter on one side of a body part and
a receiver on the other side to detect differences in attenuation
(like X-ray imaging). These systems determine
ultrasound attenuation and ultrasound speed through the
intervening tissues. One version of this system used an
acousto-optical holographic system to form the image.
Several systems have been explored to insonify tissue
from one direction and gather the scattered ultrasound
from another direction. Such systems usually require an
array of receiving transducers.
The only ultrasound systems that have gained wide
acceptance in clinical use are: (1) the continuous-wave
(CW) backscatter Doppler system, and (2) common-axis
pulse-echo backscatter imaging and Doppler systems.
The reason for the failure of the other systems is due to
two features of medical ultrasound examination: refraction
of ultrasound in tissue and the cost of receiving and
processing data from array receivers. The CW methods
are used only for Doppler measurements. The first of
these systems was constructed before 1960 [21.45, 46];
their application in medical diagnosis was well estab-
Medical Acoustics 21.4 Methods of Medical Ultrasound Examination 857
oscillation frequencies near diagnostic frequencies. The
bubbles therefore oscillate in response to incident ultrasound.
The bubble oscillation stores some energy,
which is reradiated at the original ultrasound frequency
as well as at harmonics of that frequency. Older ultrasound
systems were, by nature, insensitive to the second
harmonic of the natural transducer frequency. By using
a transducer with an increased bandwidth (3 MHz center
frequency transducer manufactured with the quality factor
reduced spreading the bandwidth to 1.9–4.1MHz)
then lowering the transmit frequency (2 MHz) and raising
the receive frequency (4 MHz), a system sensitive to
the harmonic emissions from the contrast bubbles can be
achieved. Surprisingly, harmonic echoes are generated
by tissues even when contrast bubbles are not present.
lished by 1970 [21.47–52]. Pulse-echo methods have
been under continuous development since 1950. The
majority of medical diagnostic imaging examinations in
the world are performed with pulse-echo instruments.
21.4.1 Continuous-Wave Doppler Systems
Continuous-wave Doppler was used in one of the earliest
ultrasound systems to be commercialized. Developed
Transmitter
Ultrasound
contact gel
Receiver
Ultrasound
transmitting
transducer
Patient
Blood
vessel
Ultrasound
receiving
transducer
Fig. 21.15 Nondirectional continuous-wave Doppler. In
a typical CW 5 MHz nondirectional Doppler, the incident
ultrasound beam is scattered by stationary tissue forming
a 5 MHz echo and by moving blood forming a 5.0004 MHz
echo. The two echoes arrive at the receiving transducer simultaneously.
By constructive and destructive interference
between these waves, the received 5 MHz signal is amplitude
modulated by the 5.0004 MHz echo. The frequency
of the amplitude modulation is the difference frequency
5.0004 − 5 MHz or 400 Hz, which is amplitude demodulated,
providing a 400 Hz audio signal to the speaker
Part F 21.4