TMS
Didactic Muse
Page 12
Imaging Ultrasound Cont.
REFLECTION COEFFICIENT
The reflection coefficient is a parameter based on the difference of
acoustic impedance between two structure. When the ultrasound beam collides with an interface, reflections are produced, and the reflections are due to
the difference of acoustic impedance; the bigger the difference the bigger
strength of the reflection. As in the water container part of the ultrasound beam
continues to the next interface. However, ultrasound imaging echo is usually
used instead of reflection.
reflection
coefficient
is used in physics
and electrical
engineering when
wave propagation
in a medium
containing sudden
changes relative
In figure 10, six structures are shown with different (Z) by which the ultrasound beam passes through; also the interfaces of these structures (I), I1,
I2, I3, I4 and I5. The magnitude of the difference of acoustic impedance is
defined by the symbol (≠), if the difference is small. (≠≠) is a medium level
and (≠≠≠≠) is a high level, so high that the entire ultrasound beam is reflected. In the first interface the difference of acoustic impedance is (Z1 ≠
Z2). As the difference is not so big, the amplitude of the echo is not high, so
small part of the beam returns to the probe, and other part keeps on to I2. In I2
the difference is (Z2 ≠≠ Z3). That`s why the echo is bigger than before. In I3 (Z2
≠≠ Z3), there is no difference of acoustic impedance, so there is no echo and the
entire ultrasound beam travels to the next interface. In I4, (Z4 = Z1) and (Z5 =
Z2), then there is the same difference of acoustic impedance of I1(Z1 ≠ Z2), so
the amplitude of the echo is the same. In I5 (Z5 ≠≠≠≠ Z6), the difference
of acoustic impedance is the biggest that`s why the amplitude of the echo
is the biggest. In this interface, the entire ultrasound beam is reflected, so
nothing passes through. This is like in the water container when the
waves collide with the end of the wall.
to an incident
wave.
As can be seen in table 1 the acoustic impedance, of the bone is too high and
the acoustic impedance of the air is too low. So the difference of acoustic impedance in the interface that is present bone or air is too big, no matter which other
structure is present. The ultrasound beam is entirely reflected and no ultrasound beam passes through this interface. There is no possibility of obtaining
an image after this
...
(Continued)