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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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