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8/6/2019 Medical Physics Ultrasound Imaging
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MedicalPhysics
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Ultrasound imaging
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2.2.18 Explain the piezoelectric effect
Ultrasound waves refers to sound wavesof frequencies above 20 kHz
Ultrasound waves are produced by atransducer that converts electrical
signals to an ultrasound beam
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2.2.18 Explain the piezoelectric effect
When certain crystalline minerals aresubjected to a mechanical force, the crystals
became electrically polarized. Tension and
compression generated voltages of opposite
polarity, and in proportion to the applied
force. This effect is called the piezoelectric
effect.
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2.2.18 Explain the piezoelectric effect
If these voltage-generating crystals areexposed to an electric field it lengthened or
shortened according to the polarity of the
field, and in proportion to the strength of the
field. This effect is called the inverse
piezoelectric effect.
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Acoustic impedance = velocity of sound in
material density of material
Z = v kgm-2s-1
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Attenuation of ultrasound
Ultrasound is attenuated with depth of travelthrough a material because
1. energy is used doing work against frictional and
viscous forces in the material
2. Scattering and partial reflection at the
multitude of interfaces that the beam
encounters
Ultrasound is attenuated exponentially andattenuation increases with frequency of the
ultrasound.
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2.2.20 Explain the principles of ultrasound imaging
An ultrasound image is generated when the pulsewave emitted from the transducer is transmittedinto the body, reflected off tissue interface andreturned to the transducer.
The transducer waits to receive the returningwave (i.e. echo) after each pulsed wave. Thetransducer transforms the echo (mechanical
energy) into an electrical signal which isprocessed and displayed as an image on thescreen.
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2.2.20 Explain the principles of ultrasound imaging
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Imaging techniques
A-mode (amplitude mode-echoranging)
http://www.absorblearning.com/media/item.ac
tion?quick=15b
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When the pulse returns to P, the reflected
pulse gives information of two measurements:
The amplitude of the reflected signal, and the
time it takes returning.
Amplitude is dependent on amount of energy
reflected.
Time is dependent on the distance from the
probe.
When this is measured, the scatterer isdisplayed with amplitude and position.
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To provide a sustained image the pulse is
repeated typically with apulse repetitionfrequency (PRF) of 1 kHz. The transducer is
pulsed for 1 s and receives echoes for 999 s.
B-mode (brightness mode imaging)
Transducer pulsed at regular intervals as with
A-mode
Ultrasound beam scans back and forth across
a two dimensional section
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The acoustic impedance of air at a certain place
is 430 kgm-2s-1 what is the velocity of air at that
place given that the density of the air is 1.29kgm-3 ?
About 30% of ultrasound that is incident at rightangles on a bone-tissue interface is reflected.
The acoustic impedance of the bone is 5.0 106
kgm-2s-1 , determine the velocity of the
ultrasound in the tissue given that the averagedensity of tissue is 1000 kgm-3 .
The attenuation of 1.0 MHz ultrasound by boneis 15 dBcm-1 , what is the half value layer of bone
for ultrasound at this frequency?