Diagnostic ultrasound ( PDFDrive )

CHAPTER 3 Contrast Agents for Ultrasound 57
35
Power in dB (relative to baseline)
30
25
20
15
10
10 µl/kg
20 µl/kg
40 µl/kg
100 µl/kg
200 µl/kg
5
0
–25
0 25 50 75 100 125 150 175 200 225 250 275 300
Time in seconds
FIG. 3.3 Arterial Blood Echo Enhancement Following an Intravenous Bolus of Optison at Increasing Doses. The peak enhancement is
30 dB, corresponding to a 1000-fold increase of echo power. Note that increasing the dose by a factor of 10 does not have the same effect on
the peak enhancement. Instead, it is the wash-out time that is increased.
TABLE 3.2 Three Types of Acoustic Behavior of a Typical Perluorocarbon, Lipid-Shelled Agent
in an Ultrasound Field
Peak Pressure
(Approximate)
Mechanical Index
(MI) at 2 MHz
Bubble Behavior Acoustic Behavior Application
<100 kPa <0.07 Linear oscillation Linear backscatter Doppler signal enhancement
enhancement
0.1-0.3 MPa 0.07-0.2 Nonlinear oscillation Nonlinear backscatter Real-time (low-MI) perfusion imaging
>0.5 MPa >0.4 Disruption Transient nonlinear
echoes
Triggered perfusion or disruptionreplenishment
low measurement
scatter ultrasound in a manner dependent on the amplitude of
the sound to which the imaging process exposes them. he results
are three broad types of bubble behavior, with three broad types
of resulting echoes (Table 3.2). he types of bubble behavior
depend primarily on the intensity (more precisely, the peak
negative pressure and frequency) of the incident sound ield
produced by the scanner. At low incident pressures (corresponding
to low transmit power of the scanner), the agents produce linear
backscatter enhancement, resulting in an augmentation of the
echo from blood. his is the behavior originally envisaged by
contrast agent manufacturers. As the transmit intensity control
of the scanner is increased and the negative pressure incident
on a bubble goes beyond about 50 to 100 kPa, which is still
below the level used in most diagnostic scans, the contrast agent
backscatter begins to show nonlinear characteristics, such as the
emission of harmonics. It is the detection of these that forms
the basis of contrast-speciic imaging modes such as harmonic
and pulse inversion imaging. Finally, as the peak pressure passes
about 300 kPa (or 0.3 MPa) and approaches the level emitted
by a typical ultrasound imaging system in conventional B-mode
imaging, bubbles produce a strong but brief echo as they are
disrupted by the ultrasound beam. his behavior forms the basis
of the most common way of quantifying perfusion. It should be
noted that in practice, because of the diferent sizes present in
a realistic population of bubbles 34 and the additional efect of
frequency, the borders between these behaviors are not sharp.
Nor will they be the same for diferent bubble types, the acoustic
behavior of which is strongly dependent on the gas and shell
properties. 35
To reiterate, three types of behavior of bubbles in an acoustic
ield have been identiied and depend on the amplitude and
frequency of the transmitted ultrasound beam. In practice, this
exposure is best monitored by means of the mechanical index
(MI) displayed by the scanner. At very low MI, the bubbles
act as simple but powerful echo enhancers. his is most useful
for spectral Doppler enhancement but is rarely used in the