Biomedical Engineering Reference
In-Depth Information
delay the signal changes dramatically. This time delay corresponds to a loca-
tion of 5.0 mm from the surface of the sample when using a value of 1000 m/s
as the ultrasound velocity in the sample. This is the same position at which
the thread was located at a depth of 5.0-mm. This shows that the detected
signal is changed only when the pulsed ultrasound wave is located at the
absorber. The shape of the signal was mainly affected by the pulse shape of
the ultrasound wave. Because the damping of the pulse ultrasound wave was
not optimized, the pulsed wave had some side lobes caused by ringing.
By scanning the sample against the ultrasound transducer head laterally,
the cross-sectional image of the sample can be obtained. Figure 2.27b shows
a surface plot of the cross-sectional image of the same sample. The correspon-
ding size of the image is 10.0 mm in depth and 2.0 mm in a lateral direction.
The height of the image is equivalent to the intensity of the scattered light.
The height level shows an ac signal that corresponds to the range between
−
40
.
0 mV and 40.0 mV. The center of the image shows one major peak that
corresponds to the location of the embedded thread.
The Effect of a Pulsed Ultrasound Wave and an Absorber.
The
effect of the pulsed ultrasound and an absorber is confirmed. Figure 2.28a is
the result which is observed the same sample by the same setup in Fig. 2.27b.
Figure 2.28b is an image resulting from an ac signal measurement in which
the same sample is observed by the same setup but without applying the
pulsed ultrasound wave to confirm the effect of the pulse ultrasound wave.
The ac signal was reduced to less than 1% compared with the ac signal that
was measured with the pulsed ultrasound wave. Consequently, the image
without the pulsed ultrasound wave had almost no structure. To verify the
validity of the significant ac modulation at the position of the absorbing
object, a sample without a thread was observed. Figure 2.28c shows a cross-
Light
Ultrasound
Absorber
Light
Light
Ultrasound
Absorber
(a)
(b)
(c)
Fig. 2.28.
A cross-sectional image of an absorbing object embedded in silicone
rubber and two reference images taken at various conditions for confirmation of the
effect of ultrasound and light
