Biomedical Engineering Reference
In-Depth Information
Sample surface
Absorber
Silicone rubber
(a)
(b)
Fig. 2.26.
An example of a sample.
a
is the side view and
b
is the top view. The
thickness of the sample is 10 mm. An absorber which is made of a silk thread is
embedded at a 5-mm depth from the sample surface
dius of curvature, so that the pulsed ultrasound wave converges to a point
located at a distance of 25.0 mm from the surface of the transducer. The dia-
meter of the transducer is 25.0 mm and a hole, which is for light irradiation
from the same direction and along the same axis, is opened with a 5.0 mm
diameter at the center of the transducer. The transducer contacts a coupler
made of silicone rubber to reduce the coupling loss of the ultrasound wave
at the surface of the sample. The silicone rubber is transparent in the visible
region, to transmit the laser beam eciently to the sample. The distance
from the transducer to the surface of the coupler is 20 mm, and the focal
point of the ultrasound wave is located at a depth of 5.0 mm in the sample.
The rear of the transducer is mounted on a rubber damper to generate the
ultrasound wave eciently. A hole 5.0 mm in diameter is also made in the
rubber damper, to transmit the laser beam for observation through the tran-
sducer. In generating an ultrasound wave, the voltage of 12.0 V peak applied
to the ceramic, which produces peak pressure of approximately 2
.
7
10
5
Pa
at the focal point of the ultrasound wave, is a rectangular wave from a pulse
generator. The pulse width is around 0.5
×
sandtherepetitionrateofthe
pulses is 1 kHz. The repetition rate was set at a suitably low value to wait
for a sucient time for the next pulse after the sample had been irradiated
with pulsed ultrasound. The ultrasound velocity of the silicone rubber in the
experiment was around 1000 m/s. With these parameters, the diameter of
the focal spot of the ultrasound wave is 0.3 mm in the full width at half
maximum (FWHM) and the depth of the focal area of the ultrasound wave
is 2.5 mm, also at FWHM.
Figure 2.26 shows an example of the samples used in this experiment.
Figures 2.26a,b are the side and top views of the sample, respectively. The
sample is made from an absorber and its surrounding material is made from
silicone rubber, which is the same material as the coupler in the transducer
head. The absorber is made of a silk thread stained with victorial blue dye,
μ
