54732029 Medical Physics Ultrasound Imaging

8/22/2019 54732029 Medical Physics Ultrasound Imaging

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Medical Physics


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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 a

transducer that converts electrical

signals to an ultrasound beam


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When certain crystalline minerals are

subjected to a mechanical force, the crystals

became electrically polarized. Tension and

compression generated voltages of opposite

polarity, and in proportion to the appliedforce. This effect is called the piezoelectric

effect.


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If these voltage-generating crystals are

exposed 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 inversepiezoelectric effect.


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2.2.19 Discuss properties of ultrasound

Requires a material medium in which totravel

Longitudinal

Can be reflected, refracted and focused

Velocity in a given material is constantindependent of frequency and wavelength

Velocity depends on the material in which ittravelsI. travels slower in materials with higher density

II. travels faster in materials of highcompressibility


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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 travel

through a material because

1. energy is used doing work against frictional and

viscous forces in the material

2. Scattering and partial reflection at themultitude 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 pulse

wave 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

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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 thetime 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 repetition

frequency (PRF) of 1 kHz. The transducer ispulsed 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 placeis 430 kgm-2s-1 what is the velocity of air at that

place given that the density of the air is 1.29

kgm-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 theultrasound in the tissue given that the average

density 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?

54732029 Medical Physics Ultrasound Imaging

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