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Ultrasonography Technology
Ultrasonography uses a probe containing one or more acoustic transducers
to send pulses of sound into a material. Whenever a sound wave encounters
a material with a different acoustical impedence, part of the sound
wave is reflected, which the probe detects as an echo. The time
it takes for the echo to travel back to the probe is measured and
used to calculate the depth of the tissue interface causing the
echo. The greater the difference between acoustic impedences, the
larger the echo is. The difference between gases and solids is so
great that most of the acoustic energy is reflected, and so imaging
of objects beyond that region is not possible.
The speed of sound is different in different materials,
and is dependent on the acoustical impedance of the material. Part
of the acoustic energy is lost every time an echo is formed.
Unlike regular sound, ultrasound can be directed into
a single direction. The echoes received by a stationary probe will
result in a single dimensional signal showing peaks for every major
material change.
To generate a 2D-image, the probe is swivelled, either
mechanically or through a phased array of ultrasound transducers.
The data is analysed by computer and used to construct the image.
In a similar way, 3D images can be generated by computer using a
specialised probe.
Some ultrasonography machines can produce colour images,
of sorts. From the amount of energy in each echo, the difference
in acoustic impedence can be calculated and a colour is then assigned
accordingly.
The frequencies used for medical imaging are generally
in the range of 1 to 10 MHz. Higher frequencies have a correspondingly
lower wavelength, and so images can have a greater resolution. However,
the attenuation of the sound wave is increased at higher frequencies,
so in order to better penetration of deeper tissues, a lower frequency
(3-5MHz) may be used.
Doppler ultrasonography
Ultrasonography can be enhanced with Doppler measurements, which
employ the Doppler effect to assess whether structures (usually
blood) are moving towards or away from the probe. By calculating
the frequency shift of a particular sample volume, for example a
jet of blood flow over a heart valve, its speed and direction can
be determined and visualised. This is particularly useful in cardiovascular
studies (ultrasonography of the vasculature and heart) and essential
in many areas such as determining reverse blood flow in the liver
vasculature in portal hypertension. The Doppler information is displayed
graphically using spectral Doppler, or as an image using colour
Doppler or power Doppler. It is often presented audibly using stereo
speakers: this produces a very distinctive, although synthetic,
sound.
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