Ultrasound

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Ultrasound - resolution
Sound waves with frequencies from 20-20,000Hz can be heard by humans. If
the sound waves have a frequency about this we cannot hear it. This is called
ultrasound. Ultrasound is used for scanning in hospitals, most commonly for
unborn babies, but also for hearts, abdomen and breasts. They allow you to
see inside the body and detection any abnormalities, such as heart defects,
cancer and also problems with babies. The eliminates the need to surgically
investigate the inside of person's body to find out what is wrong with them.
Ultrasound works by the fact that sound waves will be partially reflected
when it comes into contact with media. As the pulse passes into the bone,
there is a great increase in density, since bone is much more dense than flesh
or organs. This changes the speed at which the sound waves travel, resulting
in a significant reflection of the energy that can be detected. The type of
tissue boundary involved in any reflection can be determined by the proportion
of energy reflected.
The ultrasound transmitting dec
ide detects how long the echo takes to
return to it, and it also detects how much energy was reflected. A computer
can also calculate the depth at which the energy was reflected and so build
up an image of the features inside the person.
The resolution of an ultrasound scan can be improved by using a smaller
wavelengths. this means that more detail can be seen such as the hands or
feet of a baby. However decreasing the wavelength means that the wave
pulse will be absorbed quicker. These two ideas contradict each other, so a
balance is needed between the two. Depending on the use of the ultrasound,

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In general the wavelength ranges from
0.075mm to 1.5mm. This means that anything closer together than the
wavelength will be seen as one item. So fingers which are 0.5mm wide in a
foetus will only be seen in an ultrasound where the wavelength is 0.5mm or
less. Using pulse-echo imaging there is an additional problem that the
resolution will be half the length of the pulse. The wavelength can be
calculated by speed x time pulse is on.…read more

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