Imaging of Tissue/Organs with Ultrasound - Biological Imaging and Sensing - page 96

which has a high absorbance around 633 nm. The diameter of the thread is
0.3 mm and the length is around 10 mm along the y -axis. In our experiment,
the location of the thread in the depth is changed. The scattering coe cient of
the surrounding material is adjusted according to the amount of Intralipid
(Teruno, Tokyo, Japan) resolved in the sample. The thickness of the sample
is 10 mm. Experiments are performed in an almost clear sample and in a
scattering sample with absorbers.

2.4.4 An Experiment for Weak-Scattering Samples

Observation of an Absorber in the Medium. Asamplewithanabsor-
bing object in an almost transparent medium is observed. The thread was
locatedatadepthof5mm,sothatthepositionofthethreadcouldbeset
at the focal point of the ultrasound. Figure 2.27a shows an example of the
axial cross-sectional signal obtained by the apparatus. The observed signal
contains an alternating component (ac) that describes the modulation of the
scattered light intensity, induced by the pulsed ultrasound wave and a direct
component (dc) that represents almost constant intensity of the scattered
light, which is caused by the silicone rubber. The signal is an accumulation
of 256 data sets, with each set containing 500 data points along the z -axis.
The longitudinal axis shows the time delay after pulse initiation from the
piezoelectric ceramic. The value of 20.0 s on the axis corresponds to the

position at the surface of the sample, estimated from the velocity of the ul-
trasound wave. After the pulsed ultrasound wave reaches the surface of the
sample, the scattered signal is detected for a period of 10.0 μ s. At the 25.0 μ s

μ

Fig. 2.27. The observed signal of an absorber in a silicone medium. a is a graph of
the temporal light intensity scattered by an ultrasound wave. b is a cross-sectional
image of an absorbing object embedded in silicone rubber