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Medical Physics
Unit Summary
Diagnostic Techniques
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Ultrasound
Endoscopes (Fibrescopes)
X-ray imaging
Computed Axial Tomography (CT)
Magnetic Resonance Imaging (MRI)
Diagnostic imaging with radioisotopes
Positron Emission Tomography (PET)
Therapeutic Techniques
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X-ray radiotherapy
Radioisotope emplacement
Some diagnostic techniques also have therapeutic
applications. These include...
 Ultrasound
 Endoscopes (fibrescopes)
Ultrasound
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Ultrasound is sound at frequencies above 20000 Hz
which is higher than can be detected by the human ear.
Sound below 20 Hz also cannot be detected by the
human ear and is called Infrasound.
Ultrasonography is the use of ultrasound waves in
medical and industrial applications.
Sound Waves
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Sound is a longitudinal wave. It carries energy but not
matter.
Measuring Sound
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The energy carried by a sound wave is measured in
Joules.
The rate at which the energy is carried is measured in
Joules per second or...
... Watts
The area over which the energy is spread defines the
intensity (Wm-2)
Ultrasound in Medical Practice
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Ultrasound is produced by supplying an
electric current to a crystal that
changes shape. This is called the
piezoelectric effect.
The sound bounces off the internal
structures of the human body and is
detected (using the piezoelectric effect
in reverse).
An image is created by measuring the
time taken for the sound to return and
the intensity of that sound.
Attenuation & Acoustic Impedance
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As a sound wave travels through a medium some of its
energy will be absorbed by the surrounding material.
This results in a reduction of the intensity of the sound and
is called attenuation.
The amount of attenuation depends on the Acoustic
Impedance of the material.
Reflection Coefficient
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At the boundary between two mediums...
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Some sound is reflected
Some is transmitted into the new medium
Some is absorbed
Reflection Coefficient
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How much sound is reflected depends on the acoustic
impedances of the two materials.
This can be calculated using the reflection coefficient.
• When the acoustic impedances are similar
there will be little sound reflected.
Advantages of Ultrasound
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By measuring the doppler shift in the reflected sound we
can monitor blood-flow.
At the sound frequencies used, diagnostic ultrasound is
very safe and so is the preferred method of monitoring
pregnancies.
Therapeutic Ultrasound
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High frequency sound waves cause heating in bones and
denser muscles. This is used to speed up metabolism and
healing of sporting and other injuries.
Very high intensity sound waves (~105Wm-2) can be used
to break up gallstones and kidney stones and treat some
tumours.
An advantage of this method is that the heat can
cauterise (seal) wounds to prevent bleeding and speed up
healing.