Biomedical Engineering Reference
In-Depth Information
and from 0 to 70 mm in the
y
and
y
directions at the same step owing to
the finite volume of the water bath. On the
z
-axis, the measurement point
was not set, to avoid damage by the laser. We assumed that the ultrasound
propagation is symmetric in the
y
and
z
directions.
Figure 2.18 shows the propagation of ultrasound at (a) 200, (b) 250, and
(c) 300
μ
s after LIB, where the nodes of the grid correspond to the mea-
surementpoints.Inthisfigure,thegridsweredistortedasthenodeswere
shifted in proportion to the ultrasound magnitude received at the measure-
ment point, and the nodes were drawn brightly.
This figure reveals that the ultrasound propagated in any direction and
that the ultrasound has little directivity. Therefore, we succeeded in trans-
mitting a spherical wave.
2.3.4
Imaging Using Laser-Induced Breakdown
We can obtain ultrasound with little ringing and little directivity by LIB.
Using this ultrasound, we constructed an instantaneous imaging system and
performed two experiments. The first experiment was performed to observe
an echo signal by LIB and to examine whether it can be used for imaging.
Figure 2.19 shows the measurement arrangement, where LIB was generated
in a water bath and a measurement object and a hydrophone were set in the
same bath. The reflected wave from a measurement object was received with
25 hydrophones with a constant step of 10 mm on the
x
-
z
plane. A steel
wrench was selected as a measurement object, so that a signal with a larger
amplitude could be received.
Figure 2.20 shows an example of the reflected waveform. In comparison
with Fig. 2.17, this waveform has more peaks, which indicates that echo
signals were received from various portions of the measurement object.
Figure 2.21 shows an example of a 3D image with a voxel size of 5 mm
×
5mm
5 mm, where (a) and (b) are images observed from different direc-
tions. The insets in this figure are photographs of the measurement object
viewed from the same direction as the reconstructed image. Comparing the
3D images with the insets in Fig. 2.21, the 3D image can be reconstructed
correctly: in particular, the corner of the measurement object can be visua-
lized, as indicated by the arrow shown in Fig. 2.21b. These results indicate
that ultrasound generated by LIB can be used for instantaneous imaging.
The second experiment was carried out to confirm whether ultrasound by
LIB is suitable for frontally wide view imaging. Figure 2.22 shows a measu-
rement arrangement where LIB was generated in a water bath and a mea-
surement object and a hydrophone were set in the same bath. Styrene balls
were set at positions 1-3. The reflected wave from a measurement object was
received at 12 hydrophones, which were located in a circle with a radius of
15 mm.
Figure 2.23 shows an example of a 3D image, which indicates that every
object could be observed wherever it was positioned. It is noted that the
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