Biomedical Engineering Reference
In-Depth Information
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Lateral resolution
Depth resolution
Focal zone
(a)
(b)
(c)
Fig. 2.30.
The system performance of the apparatus.
a
shows the lateral resolution
of 0.3 mm.
b
shows the axial resolution of 1.5 mm.
c
shows the focal zone of the
focusing ultrasound wave along the
z
-axis
The Performance of the Apparatus.
The performance of the develo-
ped system has been estimated. Figure 2.30a shows the lateral cross-section
of Fig. 2.28a at the 5 mm location. In defining the lateral resolution as a
FWHM of the cross-section, the value is obtained as 0.3 mm. This value is
reasonable from the expected diameter of the ultrasound wave. Figure 2.30b
shows the resolution along the
z
-axis, which is estimated as 1.5 mm from
themeasurementatFWHMoftheenvelopeoftheacsignal.Theenvelopeis
fitted by the Gaussian distribution. The shape of the signal was mainly affec-
ted by the pulse shape of the ultrasound wave, because the damping of the
pulsed ultrasound wave was not optimized, so that the pulsed wave had some
side lobes caused by ringing. To decrease the FWHM in the depth direction,
the observed signal is deconvoluted with the shape of the ultrasound, because
the FWHM in the depth direction is determined from the convolution of the
pulse shape of the ultrasound and the size of the absorber. Figure 2.30c is the
superposed trace of each raw data value in Fig. 2.29. The envelope of the raw
data exhibits the axial response of the apparatus, which is mainly determined
by the change in amplitude of the ultrasound wave as it propagates in the
axial direction. The FWHM of the envelope was 4.2 mm.
2.4.5
An Experiment in a Strongly Scattering Medium
A sample that gave strong light scattering was observed. The sample consisted
of silicone rubber that contained Intralipid, to simulate a highly scattering
medium. Intralipid was dispersed into the silicone rubber with a surfactant.
The scattering coe
cient of the silicone rubber was measured at 2.0 cm
−
1
by using the Beer-Lambert law. An absorber was embedded at a depth of
4.5-mm from the surface of the sample. Figure 2.31a shows the axial cross-
sectional signal of the sample. The lateral axis shows an ac signal level, which
