Definition of Ultrasound
Sound with frequency greater than 20,000 cycles per second or 20kHz. Audible sound sensed by the human ear are in the range of 20Hz to 20kHz.
Advantages:
Ultrasound can be directed as a beam.
Ultrasound obeys the laws of reflection and refraction.
Ultrasound is reflected by objects of small size.
Ultrasound obeys the laws of reflection and refraction.
Ultrasound is reflected by objects of small size.
Disadvantages:
Ultrasound propagates poorly through a gaseous medium.
The amount of ultrasound reflected depends on the acoustic mismatch.
The amount of ultrasound reflected depends on the acoustic mismatch.
The Four Acoustic Variables:
1. Pressure - the amount of force over a given area.
2. Distance - particle displacement with the wave
3. Temperature -
4. Density
Reflection and Propagation:
Effect of propagation through gaseous zones - poor propagation, inadequate imaging.
Effect of propagation through dense zones - nearly all of the US is reflected. Structures below dense zones are poorly imaged.
Examples of dense materials - bone, calcium, metal.
Examples of dense materials - bone, calcium, metal.
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Material
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Speed of Propagation
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|
bone
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4080 m/s
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|
blood
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1570 m/s
|
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tissue
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1540 m/s
|
|
fat
|
1450 m/s
|
|
air
|
330 m/s
|
Definitions:
Cycle - the combination of one rarefaction and one compression equals one cycle.
Amplitude - the maximum displacement of a particle or pressure wave.
Intensity - the amount of force or energy of sound.
Decibel (dB) - a numerical expression of the relative loudness of sound.
Wavelength - the distance between the onset of peak compression or cycle to the next.
Velocity - the velocity is the speed at which sound waves travel through a particular medium. Velocity is equal to the frequency x wavelength.
Wavelength - the distance between the onset of peak compression or cycle to the next.
Velocity - the velocity is the speed at which sound waves travel through a particular medium. Velocity is equal to the frequency x wavelength.
The velocity of US through human soft tissue is 1540 meters per second.
Frequency - the number of cycles per unit of time. Frequency and wavelength are inversely related. The higher the frequency the smaller the wavelength.
Acoustic Impedance - simply put, acoustic impedance is dependent on the density of the material in which sound is propagated through. The greater the impedance the more dense the material.
Frequency - the number of cycles per unit of time. Frequency and wavelength are inversely related. The higher the frequency the smaller the wavelength.
Acoustic Impedance - simply put, acoustic impedance is dependent on the density of the material in which sound is propagated through. The greater the impedance the more dense the material.