TMS
Didactic Muse
Page 11
Imaging Ultrasound Cont.
The electric stimulus applied to the crystal is converted into a mechanical
oscillatory movement, and as it happened in the water container with the waves
in the water, these mechanical oscillatory movements generate ultrasound
waves that travel through the human body (fig 7). The oscillatory movements of
the crystal are in the range of Megahertz, usually from 1 to 7.5 Megahertz used
in diagnostic ultrasound imaging.
Voltage applied
produces an oscillatory
movement of the crystal.
The way of reception of ultrasound waves in the human body is done in
a similar way to that in the water container when the waves collide with the
meshes. The ultrasound waves collide with different structures inside the
body called interfaces. Part of the ultrasound waves go through the interfaces and part return to the crystals of the probe. When the waves collide
with the crystal, they cause it to compress; the crystal shrinks, after the
pressure disappears, it oscillates until stay steady. The crystal has another
important property. If it changes its size, there appears a potential difference between its ends. So the oscillating movement generates an alternating
voltage in its ends. This is the source for producing the ultrasound image.
ACOUSTIC ULTRASOUND IMPEDANCE (Z)
Acoustic impedance (Z, Rayls), is the property the biological structures
have to oppose to the ultrasound waves that go through them, and depends on
the density and elasticity of the structure. In the water container the reflection were due to collisions of the waves with the solid part of the mesh. In the
biological tissues, the interfaces between the structures acts as the meshes,
and the collisions with the interfaces produces reflections (fig. 9).
Mechanical oscillatory
movements generates
ultrasound waves that
travels through the
human body.