Biomedical Engineering Reference
In-Depth Information
thermal strain, variations in ultrasonic attenuation, and
changes in backscattered ultrasonic energy that occur due to
thermal effects. This chapter is an extension of a review article
published in 2005.
41
That article focused on the hyperther-
mia temperature range. We examine approaches that appear
to have the greatest potential for monitoring temperatures in
both the hyperthermia (41-45°C) range and in ablation bor-
der zones ( >60°C).
The rectangular region in the RF images of Figure 13.1 high-
lights an image feature that appears to move as the specimen
was heated. Movement in the axial direction in images is simi-
lar to the shifts in the A-mode echo signals. Apparent motion
toward the transducer is consistent with the change in SOS in
the water bath. The images show, however, that there is also an
apparent lateral movement, presumably due to changes in the
tissue alone.
There are also clear changes in backscattered signal strength
with temperature. This effect is seen in the delineated bands of the
A-mode signals in Figure 13.1, which isolate scattering regions
whose energy increased (band #1) or decreased (band #2) with
temperature.
13.2 thermal Effects on Backscattered
Ultrasound
As tissue is insonified during heating, at least two effects are eas-
ily seen in ultrasonic backscattered signals and images. They are
a shift in apparent position of scattering regions and changes in
signal strength from those regions. These changes are associated
with thermal effects on SOS in tissue and on tissue attenuation
and backscatter-coefficient properties.
13.2.2 Nonthermal and Unwanted
thermal Effects
To rule out nonthermal sources in backscattered signals and
images, changes in ultrasonic signals and images must be deter-
mined over the duration of a measurement paradigm when no
heating occurs. Unwanted thermal effects include those on the
measurement system itself. Spurious thermal effects may be seen
in tissue as well. For example, in echo shift measurements, the
heated region has a tendency to cause a “thermal lens” effect that
distorts the image of the tissue beyond the heated region, where
this effect can cause artifacts in temperature estimation.
44
Thermal effects on a typical phased array transducer in a
water bath have been seen in B-mode images of the stainless
steel wires in the AIUM 100 mm test object.
45
Apparent motion
toward the transducer occurred with an increase in tempera-
ture during heating from 37 to 50°C.
43
That motion, however,
can be accounted for by the change in SOS in the water bath.
The changes in backscattered signal level due to thermal effects
13.2.1 Echo Shifts and Changes
in Signal Strength
Amplitude-mode backscattered signals and radio-frequency
(RF) images from samples of bovine liver during uniform heat-
ing in a water bath are shown in Figure 13.1. With temperature
increase, the time it took for echoes from the liver specimen
interfaces and scattering regions to reach the insonifying trans-
ducer changed in the A-mode echo signals. Apparent motion in
the RF images occurred in part because SOS changes with tem-
perature. SOS, however, was assumed fixed at 1540 m/s by the
imaging system.
Echoes from a fixed view of bovine liver
#1
#2
RF images from a fixed view of bovine liver
50°C
corrected
50°C
25
37°C
50°C
20
0.5
0.5
0.5
1
1
1
15
1.5
1.5
1.5
2
2
2
10
2.5
2.5
2.5
5
3
3
3
3.5
3.5
3.5
0
37°C
2
4
6
246
246
0
5
10
15
20
Time (µs)
Lateral (mm)
FIGURE 13.1
(
See color insert.
) Changes in backscattered ultrasound with temperature. (Left) Echoes measured from a single site in a 1 cm
thick sample of fresh bovine liver at temperatures from 37 to 50°C. The two delineated echoes (indicated by bands marked #1 and #2) shift with
temperature and have energies that appear to change with temperature (similar to Figure 13.4 in Arthur et al.
42
). (Right) RF images of a fixed region
in bovine liver showing apparent motion from 37 to 50°C. The right panel shows the image at 50°C after motion compensation (similar to Figure
13.3 in Arthur et al.
43
).
