Diagnostic ultrasound ( PDFDrive )

CHAPTER 3 Contrast Agents for Ultrasound 59
Bubble diameter (µm)
10
9
8
7
6
5
4
3
2
1
1
2 3 4 5 6 7 8 9 10
Frequency (MHz)
FIG. 3.4 Microbubbles Resonate in a Diagnostic Ultrasound Field.
This graph shows that the resonant—or natural—frequency of oscillation
of a bubble of air in an ultrasound ield depends on its size. For a 3.5-µm
diameter, the size needed for an intravenously injectable contrast agent,
the resonant frequency is about 3 MHz.
Echo amplitude (dB)
0
–10
–20
–30
–40
–50
–60
–70
–80
Fundamental
Harmonics
0 5 10 15 20 25
US frequency (MHz)
FIG. 3.6 Harmonic Emission From Optison. A sample of a contrast
agent is insonated at 3 MHz and the echo analyzed for its frequency
content. The largest peak of the energy in the echo is at the 3-MHz
fundamental, but there are clear secondary peaks in the spectrum at 6,
9, 12, 15, and 18 MHz, as well as peaks between these harmonics
(known as “ultraharmonics”) and below the fundamental (known as the
“subharmonic”). The second harmonic echo is about 18 dB less than
that of the main, or fundamental, echo. Harmonic imaging and Doppler
aim to separate and process this signal alone. (With permission from
Becher H, Burns P. Handbook of contrast echocardiography: left ventricle
function and myocardial perfusion. New York: Springer; 2000. 63 )
Pressure (kPa)
100
0
–100
Radius (µm)
µm
4
2
0
4
2
0
–2
–4
4
2
0
2
µm –2
0
–2 µm
–4 –4
0 2 4 6 8 10 12
0 2 4 6 8 10 12
Time (µs)
FIG. 3.5 A Microbubble in an Acoustic Field. Bubbles respond
asymmetrically to diagnostic sound waves (top graph), stiffening when
compressed by sound, yielding only small changes in radius (bottom
graph). During the low-pressure portion of the sound wave, the bubble
stiffness decreases and radius changes can be large. This asymmetric
response leads to the production of harmonics in the scattered wave.
4
understanding of sound on which ultrasound imaging is based,
was irst led in 1917 to investigate this by his curiosity over the
creaking noises that his teakettle made as the water came to a
boil. 39 he consequence of such nonlinear motion is that the
sound emitted by the bubble, and detected by the transducer,
contains harmonics, just as the resonant strings of a musical
instrument, depending on how they are bowed or plucked, will
produce a timbre comprising overtones (the musical term for
harmonics), exact octaves above the pitch of the fundamental
note. he origin of this phenomenon is the asymmetry that begins
to afect bubble oscillation as the amplitude becomes large. As
a bubble is compressed by the ultrasound pressure wave, it
becomes stifer and hence resists further reduction in its radius.
Conversely, in the rarefaction phase of the ultrasound pulse, the
bubble becomes less stif, and therefore enlarges much more
(Fig. 3.5). Fig. 3.6 shows the frequency spectrum of an echo
produced by a microbubble contrast agent ater exposure to a
3-MHz burst of sound. he particular agent is Optison, though
most microbubble agents behave in a similar way. Ultrasound
frequency is on the horizontal axis, with the relative amplitude
on the vertical axis. In addition to the fundamental echo at
3 MHz, a series of echoes occur at whole multiples of the transmitted
frequency, known as higher harmonics. Here, then, is one
simple method to distinguish bubbles from tissue: excite them
so as to produce harmonics, and detect these in preference to
the fundamental echo from tissue. Key factors in the harmonic
response of an agent are the incident pressure of the ultrasound
ield, the frequency, the size distribution of the bubbles, and the
mechanical properties of the bubble capsule (a stif capsule, for
example, will dampen the oscillations and attenuate its nonlinear
response).