TMS
Didactic Muse
Page 18
Imaging Ultrasound Cont.
Amplitude of the
ultrasound beam
decaying exponentially
in time and the echo
is amplified in reverse.
is dependent on the difference of acoustic impedance of the interfaces
that produce the echo. Let’s go back to the water container. When the waves
collide with the first mesh, part of it returns to the cork, and the other go
through the first mesh. The part of the waves that go through cannot be
of the same amplitude as the ones that collided with the first mesh, so the
amplitude is smaller and it happens each time the waves collide with a mesh.
As it is colliding, it keeps on losing amplitude and the reflections are
smaller. It also happens to the ultrasound beam. As long as the beam is traveling it is suffering attenuation and the amplitude is decaying exponentially
as it goes deeper in the body. It also can be said, that it is decaying exponentially with the time because more elapsed time means that the ultrasound
beam travels deeper. The decaying is because of the reflections, refractions
and frictions with the molecules of the tissues. (fig. 20) has the same structure as fig. 10, and also the same differences of acoustic impedances,
but in (fig. 20) is taken into account the attenuation of the ultrasound beam.
The interfaces, I1 and I3 have the same differences of acoustic impedances,
so both have to produce echoes of the same amplitude, but on account
of the attenuation it is not possible and the echo produced at I3 is smaller and
on screen appears as wrong information of the echo. It appears on screen as
points with different brightness because of the difference of amplitude between them coming from equal reflection coefficient interfaces, and the two
points ought to be of equals brightness. The problem cannot be solved by supplying bigger power to the beam in the transmission because attenuation is
always present and the farther interface will always receive the ultrasound
beam with more attenuation than the interfaces near the probe. The solution is in the reception process. As the echoes from I1 and I3 must have the
same amplitude, and is impossible to obtain that equality at the interfaces, it
is necessary to amplify more the echo form I3 than the one from I1 so
the difference of amplitude applied to the echo from I3