What is Ultrasound?
Ultrasound (US) is high frequency sound (pitch) which is above the audible range of human hearing.
The audible range for humans varies with age but is roughly between 15 and 20,000Hz (20KHz). Canine hearing has a much greater range and at the higher end can detect sounds as high as 65,000 Hz (65KHz)
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Sound is simply the transfer (or propagation) of mechanical energy from a vibrating source through a medium (e.g., air, water, soft tissue)
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A vibrating source is necessary to produce a pressure or ‘sound’ wave
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Sound cannot travel in a vacuum because there are no particles of matter to interact with each another to ‘pass on’ the sound vibrations
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Diagnostic ultrasound range is between 2—20MHz with a typical ultrasound probe operating in the region of 5Mhz
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An ultrasound probe or ‘transducer’ creates ultrasound when it converts electrical energy into mechanical energy and vice versa
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The transducer is designed to produce ‘steerable’ narrow beams of sound which are composed of multiple fine individual scan lines which can be represented on our screen in the form of a pixel matrix (picture grid pattern) where each pixel has a signal which is generated from a sound pulse returning from a reflective interface within the patient’s body
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Returning signals are ‘plotted’ on the screen by the computer in the ultrasound equipment and each signal corresponds to the appropriate depth and strength of the returning sound pulses and a real time ‘map’ of the body area which has been insonated is built, frame by frame
Ultrasound Generation
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Ultrasound waves are mechanical, longitudinal waves which are generated by numerous tiny piezoelectric (PZT) crystals situated behind the front face of the transducer material
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‘Crystal’ is the scientific name for any solid with atoms/molecules arranged in an orderly way
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Basically, the properties of PZT crystals mean that squeezing or compressing them will generate electricity (piezo comes from the Greek for squeeze or press) and the reverse is true – pass electricity through the crystals and they will ‘squeeze’ themselves
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Applying an alternating electrical signal causes the crystal to repeatedly expand and contract with the alternating voltage
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The expansion/contraction cycle causes the vibrations which are needed to generate an ultrasound pulse (which then corresponds to a scan line on our screen)
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The frequency of the voltage applied determines the frequency at which the material vibrates and consequently the frequency at which the ultrasound pulses are generated
Ultrasound detection
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Sound returning from tissue interfaces applies a pressure or force to the face of the PZT crystal which causes the crystal to expand and contract and consequently vibrate. It is these vibrations which create electricity which is proportional to the force applied
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The frequency of the force applied to the crystals will affect the frequency of the voltage generated
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The stronger the returning force applied, the greater the amplitude of the electrical signal generated so a strongly reflecting interface such as bone will cause a high amplitude signal
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The force generated by a weakly reflecting substance such as urine or circulating blood creates a smaller force at the probe surface and therefore a low amplitude signal
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The signal representing bone or mineralisation will be displayed as hyperechoic or bright / white on the screen and the signal from blood or urine will appear as hypoechoic or dark/black on the screen
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Tissue reflections creating amplitudes between the extremes of black and white will appear as different shades of grey, hence ‘grey scale’
Historically, 2-dimensional ultrasound scanning was known as B- Mode (Brightness-modulated) because the amplitude of the electrical signal generated, correlates to the brightness of the signal displayed on the screen. Many ultrasound machines have a B-mode control which you can use if you have used lots of controls when optimising your image and just want to go back to where you started or you have been using doppler, m-mode or power doppler and want to go back to ‘basic’ scanning