Medical Physics: Ultrasound

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  • Created by: LZ95
  • Created on: 14-04-14 16:19
What is an ultrasound?
A sound that is higher than the upper limit of human hearing
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What does transducer do?
Generates and detects ultrasound waves
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What happens when you squeeze a crystal of quartz?
A voltage is induced
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What happens when you apply voltage to a piezoelectric material?
It changes shape
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What happens to crystals when a voltage is induced?
They vibrate
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What does the backing material in the transducer do?
Damps out oscillations
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When is the maximum amount of energy achieved?
When the crystal is in resonance
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What sort of wave is ultrasound when it enters the body?
It is a longitudal wave
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What happens to ultrasound once it is in the body?
It is reflected at boundaries between different tissues and is absorbed by tissues
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Why can't the probe be placed straight onto the skin?
Nearly all the energy would be reflected
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What is usually used as a coupling medium?
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What is the definition of the resolution of the beam?
Smallest distance that can be discriminated in the image.
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How does frequency affect resolution?
The higher the frequency, the higher the resolution
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What is axial resolution?
Resolution in the direction of the beam
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How is axial resolution improved?
By making pulses short
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What is lateral resolution?
Resolution determined by the beam width.
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What does the electrical transducer do to returned pulses?
Turns them into electrical signals
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Name 3 ways in which the signals can be stored.
1. Video tape, 2. storage CRO, 3. Digital data
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What is an A scan?
Amplitude modulated display
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What is a B scan?
Brightness modulated display
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Name 3 advantages of ultrasound scans
Non-invasive, no discomfort, no hazards for operator, equipment relatively cheap
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Name 3 disadvantages of ultrasound scans
Sonographer has to be skilled, image needs skillful interpretation, attenuation reduced resolution of image
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What is attenuation?
The reduction of intensity as the wave travels through a material
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Name 2 dangers of ultrasound scans.
1. Temperature rise in tissues 2. Pressure changes can rupture cells
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When does an ultrasonic wave have its largest amplitude for a given impact of energy?
When the a.c. signal matches the natural frequency of the crystal (they are in resonance)
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What is specific acoustic impedance (Z)?
Product of density and the speed of sound
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What is the reflection coefficient when a plane wavefront strikes the boundary between two materials at normal incidence?
coefficient = I(R)/I(i) where I(R) = intensity of reflected wave and I(i) is the intensity of the incident wave
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What is another way to write this?
((Z2-Z1)/(Z2+Z1))^2 where Z1 and Z2 are the specific acoustic impedances of the two media
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What are used to build an image of the internal structure of the body?
'Echoes' from different boundaries
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What equation gives the intensity of the transmitted wave?
I(T) = I(i) - I(R)
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What is the equation for the shortest possible time for 1 pulse in a B scan?
Tmin = distance/speed = 2Dmax/1500 where Dmax = max depth imaged and 1500=speed of sound in tissue
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What does the reciprocal of this equation give?
Number of frames per second (Hz)
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What is the output of an A scan?
Graph on an oscilloscope (change in echo amplitude vs time)
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What happens in a B scan as the beam sweeps across the body?
There is a rapid sequence of A scans
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What controls the brightness of the picture?
Strength of echo signal
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When does refraction happen?
When rays strike the inside surface of a glass fibre at less than critical angle
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What happens if fibres touch each other in a bundle?
Light leaks from one fibre to the next
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How is this prevented?
Fibres are coated with a second layer of glass of slightly lower refractive index
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How does the cladding help?
It increases the critical angle at the edge of the fibre and also protects the core from surface scratches
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What are the advantages of MR scanning?
It is very safe as the patient is not subjected to ionising radiation and they produce more detailed images of soft tissue than CT scans
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What are the disadvantages of MR scanning?
MR scanning takes longer, requires patient to be still for longer, more expensive than CT scans and patients may find it claustraphobic
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What form the nucleus of every hydrogen atom in the body?
A proton
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How do protons generate their own magnetic field?
They spin around
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What do MR scans use to get images?
Superconducting magnets
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What is directed at the patient which disturbs the orientation of protons?
A pulse of radiowaves
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What do protons do once they return to their original state?
Emit a radio frequency signal
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Other cards in this set

Card 2


What does transducer do?


Generates and detects ultrasound waves

Card 3


What happens when you squeeze a crystal of quartz?


Preview of the front of card 3

Card 4


What happens when you apply voltage to a piezoelectric material?


Preview of the front of card 4

Card 5


What happens to crystals when a voltage is induced?


Preview of the front of card 5
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