Imaging of Tissue/Organs with Ultrasound - Biological Imaging and Sensing - page 100

Fig. 2.31. The observed signal for a scattering medium. a is a graph of the temporal
light intensity scattered by an ultrasound wave. b is a surface plot of the cross-
sectional image of an absorbing object embedded in silicone rubber

ranged from 3 . 5 mV to 3.5 mV. At 4.5 mm from the sample surface, the

modulated signal caused by the absorbing object appeared strongly. The am-
plitude of the ac signal is smaller compared to the signal obtained from the
sample without Intralipid in Fig. 2.27a. This is due to the fact that the scat-
tered light modulation generated by the pulsed ultrasound wave decrease as
the scattered light propagates from the absorber through the sample. Fi-
gure 2.31b shows a cross-sectional image of the sample. The height indicates
the ac signal level of the scattered light. In this result, a peak of the signal
has been observed, which corresponds to the location of the absorber.

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2.4.6 Conclusions

In this section, a detailed description of the imaging of tissues by an ultra-
sound wave and light is given. The basic physical mechanism leading to the
change of the optical scattering pattern due to a pulsed ultrasound wave has
been explained. A new transmission system for optical measurement has been
developed, and the system performance has been assessed. Furthermore, this
system have shown the cross-sectional image of an optical absorber embedded
in scattering medium. From the results, this system shows the possibility of
depth discrimination of an absorbing.
The technique described is able to be extended to the imaging of blood
vessels under a scattering medium such as skin tissue, and is therefore an aid
in the study of biological structures. This system limits the thickness of the
sample at around a few centimeters, because the incoming light needs to be
transmitted through the tissue sample. The proposed technique may possi-