Biomedical Engineering Reference
In-Depth Information
microcapsule is essential to achieve measurement and control of the system
in the blood flow [45].
However, with a conventional imaging technique, echography must be
used, which provides an insucient spatial resolution of 1-3 mm. The Dopp-
ler method only provides the flow velocity component projected into the
ultrasonic beam.
To alleviate these constraints, a two-dimensional moving target indication
(2D-MTI) method has been developed to enable direct visualization of the
2D vector and path line (streamline) of a microcapsule in the blood flow.
The principle of the 2D-MTI method is serial image accumulation of sub-
tracted images (high-speed DSE images) of echoic particles in blood like
optical flow method. In the DDS system, the high-speed DSE system [37]
executes image subtraction between successive frames of high-speed B-mode
echograms of the blood flow containing highly echoic particles, such as micro-
capsules.
The digital subtraction unit reveals the new location of an echoic micro-
capsule as a positive intensity (white), on its 2D high-speed DSE image and
displays the vanishing location as a negative intensity (black). Finally, the
accumulated high-speed DSE image shows the streamline; that is, the trace
ofrapidmotionofthemicrocapsule.
Through the 2D-MTI method, the path lines of rapid motion in vitro
were demonstrated with highly echoic particles, air-filled microcapsules made
of PMMA. The current 2D-MTI system was able to visualize the path of
microcapsules smaller than the lung capillaries of 8
μ
m diameter without the
restraints of Doppler echography.
2.5.4
Collapse Monitoring of Microcapsules
The brightness of the microcapsule suspension captured on the echogram
decreases after ultrasound emission, because microcapsules collapse at their
resonant frequency. Therefore, by comparing two successive echograms, the
location and degree of microcapsule collapse can be indicated [46].
To evaluate the degree of microcapsule collapse, we applied a method to
measure the density of capsules from the B-mode echogram. This method elu-
cidated the relation between the density of the capsules and in the brightness
of the echogram. The variation in the density of the capsules is calculated
throughout digital processing of successive B-mode echograms. We construc-
ted in vitro phantom with fixed microcapsules and calculated the density
of the capsules. Figure 2.32 indicates the experimental setup to capture the
echogram of capsules inside a phantom [47].
Figure 2.33 shows an echogram of a cross-section of a phantom. Inside
the phantom, capsules can be seen as speckles. From the brightness variation
in proportion to the depth, the density of the capsules can be estimated.
The brightness
B
depends on the depth
x
by calculation of the constant
