Biomedical Engineering Reference
In-Depth Information
on the AIUM wires and transducer, however, were small. The
change in signal level from a wire in the AIUM test object over
one hour was <±0.1 dB at a constant temperature. Images at 37
and 50°C showed a change in signal level of about 0.3 dB. The
small changes over time and with temperature due to effects on
the test object and transducer are usually negligible compared to
thermal effects in tissue.
We also tested our system under conditions for which we
expected to see no change in backscattered energy. We mea-
sured the energy in backscattered signal levels in bovine liver
over the time required for an experiment, but without heating,
to determine the magnitude of nonthermal effects. Signal-level
changes of <±0.2 dB were typically seen over 80 minutes, twice
the approximate duration of an experiment. 42
Quantifying the time dependence of echo positions is important
because echo shift is the basis for much of the recent work on
temperature estimation. In addition, in order to compare sig-
nal strengths under consistent conditions, motion of scattering
regions must be tracked so that compensation for that motion
can be applied as shown in Figure 13.2.
Techniques for motion estimation in ultrasonic images, that
is, for estimation of the displacement between two image regions,
comprise an active field of research 47-51 and have been used suc-
cessfully for elasticity imaging, phase aberration correction,
blood velocity estimation, and other applications. In these areas,
motion estimation is typically called speckle tracking or time-
delay estimation. Work based on exploiting thermal effects that
induce tissue strain has been aimed at guiding focused ultra-
sound therapies, as well as estimation of temperature. 52-55
Measurement of echo shifts and the tracking of real and
apparent motion has been based on correlation techniques.
Employing RF signals permits the use of cross-correlation as a
similarity measure for automatic tracking of regions as a func-
tion of temperature. The result of maximizing 2D cross-correla-
tion to estimate displacement and then shifting and resampling
images to apply that displacement is shown in the RF images of
13.3 Image Motion
Tracking the echoes from tissue interfaces and scattering
regions is important for any temperature estimation method
because a key challenge for in vivo studies is measurement of
thermal effects in the presence of real motion in a perfused liv-
ing system, in addition to apparent motion from thermal effects.
Image @ 37°C
Image @ 41°C after motion compensation
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Image @ 37°C
Image @ 45°C before motion compensation
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Image @ 41°C before motion compensation
Image @ 41°C after motion compensation
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FIGURE 13.2 (Left) Conventional B-mode images at various temperatures from a specimen of turkey breast muscle before motion compensa-
tion. Apparent motion can be seen clearly in the boxes. (Right) Conventional B-mode images at the same temperatures after motion compensa-
tion. The patterns isolated in the boxes show both motion compensation and a change in the signal strength with temperature. (From Guo, Y., A
Framework for Temperature Imaging using the Change in Backscattered Ultrasonic Signals, PhD thesis, Washington University, St. Louis, MO, 2009.
With permission.)
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