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Unit 20 Assignment 2
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Unit 20 Medical Physics Techniques Assignment 2 (P M D)
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Laura Latkova 2.1, 2.1Ultrasound
Ultrasound is formed of high frequency sound waves which a human ear cannot hear.
Animals such as dolphins or bats can hear it, in fact they use it to locate its prey by using
echolocation.
Ultrasound is produced when an electric current is
applied to transducer surface with piezoelectric
crystals, it starts to vibrate creating sound waves at
high frequency.
Ultrasound waves are reflected by different
materials in air or liquid. This is called echo.
Acoustic impedance of materials is how resistant it
is when ultrasound is applied; how much it is reflected or refracted by material when
ultrasound is applied. It all depends on density of material. By using acoustic impedance
and the time it took for the ultrasound to travel back we can calculate the distance by using
this formulae distance= speed x time. The acoustic impedance can be calculated by using
the equation Z (acoustic impedance) = p (density of material) x V (acoustic velocity)
Pulse-echo technique is used in ultrasound of foetuses or organs. The device which send the
ultrasound measures the time taken for the echo to travel back. This calculates the distance
and can produce an image of soft tissue.
Sonar principles are used in marine. They use same technique of pulse echo technique; they
send ultrasound and when there are fish or submarine the echo will be reflected and will
come in two echo waves (the earlier will be fish or submarine the second will be ocean
floor; if only one echo comes back there is only ocean floor). They can also use this to map
the ocean floor and the depth of ocean.
A-scan plots a one dimensional graph which peaks show the depth of the ultrasound into
body. It is usually used to scan eyes. B-scan created two dimensional graph with dots of
different brightness; the brightness of a dot represents the strength of signal reflected from
organ. M-scan is series of B-scans which produces real motion of organs. This is commonly
used for scanning foetuses and organs to check there functioning.
Doppler Effect is the change in sound wave frequency when an object is moving towards
you and away from you. This can measure the speed and it is used in measuring blood speed
in vessels.
X rays
X rays are waves with very short wavelengths and high energy. They are produced when an
electron collides with an atom of metal target. The energy loss of an electron colliding is
transformed into 99% heat and only 1% X rays.
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