Ultrasound is sound waves with a frequency above the human hearing range of 20 000 Hz (20 kHz).

What type of wave is ultrasound?

Ultrasound is a longitudinal wave.

An A-scan (amplitude scan) is a one-dimensional ultrasound scan used to determine the distance or depth of an internal structure.

A B-scan (brightness scan) is used to build up a two- or three-dimensional image of an internal structure, using a number of sensors or one sensor in different positions.

Why are smaller wavelengths of ultrasound used to produce clearer B-scan images?

Smaller wavelengths allow the ultrasound waves to diffract around finer points of detail on the structure, giving more detailed images.

An A-scan gives a .......... only and does not produce an image.

An A-scan gives a measurement only and does not produce an image.

True or False? An A-scan produces a two-dimensional image of an internal structure.

False. An A-scan gives a distance or depth measurement only; it is the B-scan that produces a two- or three-dimensional image.

Why are pulsed, rather than continuous, ultrasound waves used in B-scanning?

Pulsing allows time for the reflected wave to be received before the next pulse is transmitted, so the transmitted and reflected waves do not interfere.

Define the piezoelectric effect .

The piezoelectric effect is the ability of particular materials to generate a potential difference (p.d.) by transferring mechanical energy to electrical energy.

Define a transducer .

A transducer is a device that converts energy from one form to another.

What happens to a piezoelectric crystal when an alternating p.d. is applied across it?

It vibrates at the same frequency as the alternating p.d., generating ultrasound waves.

Why must a piezoelectric crystal be cut to a specific size?

So that it vibrates at its resonant frequency when an alternating p.d. is applied.

Why is the piezoelectric crystal in an ultrasound transducer heavily damped, for example with epoxy resin?

To stop the crystal vibrating for too long, producing short pulses and increasing the resolution of the ultrasound device.

A common piezoelectric material used in ultrasound transducers is .......... , made from a lattice of silicon dioxide atoms.

A common piezoelectric material used in ultrasound transducers is quartz , made from a lattice of silicon dioxide atoms.

True or False? A piezoelectric crystal can only transmit ultrasound, not receive it.

False. The same crystal can act as both a receiver , converting sound waves into an alternating p.d., and a transmitter , converting an alternating p.d. into sound waves.

How does a piezoelectric crystal detect a reflected ultrasound pulse?

The returning pulse causes the crystal to vibrate, which generates an alternating p.d. across the crystal that can be processed for diagnosis.

The greater the difference in acoustic impedance between two media, the .......... the proportion of ultrasound reflected at the boundary.

The greater the difference in acoustic impedance between two media, the greater the proportion of ultrasound reflected at the boundary.

Why does sound travel faster through solids than through gases?

Particles are closer together in solids, so vibrations can be passed between them more quickly than in a gas.

Define impedance matching .

Impedance matching is when two media have a similar acoustic impedance, resulting in little to no reflection of the ultrasound wave at their boundary.

Why is coupling gel used between an ultrasound transducer and the skin?

Gel has a similar acoustic impedance to skin, so this impedance matching minimises reflection and maximises transmission of ultrasound into the body.

True or False? Air and soft tissue are well impedance matched, so ultrasound passes easily from air into the body.

False. Air and soft tissue have very different acoustic impedances, so almost all the ultrasound is reflected at an air-skin boundary; coupling gel is needed to impedance match.

What property of an ultrasound wave increases the resolution of the resulting image?

A higher frequency (shorter wavelength) increases the resolution.

What two pieces of information does a reflected ultrasound pulse give about a tissue boundary?

Depth : from the time delay between transmission and receipt of the pulse. Nature : from the amount of transmitted/reflected intensity received, which varies with tissue type.

Where in the body does ultrasound get reflected?

At boundaries between different tissues, for example between fluid and soft tissue, or tissue and bone.

Ultrasound echoes are detected by the transducer and converted into .......... signals, which are sent to the scanner.

Ultrasound echoes are detected by the transducer and converted into electrical signals, which are sent to the scanner.

What component of the transducer detects the reflected ultrasound pulse?

The piezo-electric crystal , which generates an electrical signal in response to the returning pulse.

True or False? A lower-frequency ultrasound wave gives a higher-resolution image than a higher-frequency wave.

False. A higher frequency (shorter wavelength) gives a higher -resolution image, allowing finer detail to be distinguished.

Define the Doppler effect .

The Doppler effect is the frequency change of a wave due to the relative motion between a source and an observer.

What happens to the observed frequency of ultrasound when the source moves towards the observer?

The frequency increases (the wavelength is shortened).

Why is Doppler ultrasound imaging effective for measuring blood flow?

Blood contains iron , which is highly reflective to ultrasound, so pulses reflected from moving blood cells can be detected.

The transducer detects an .......... in frequency if the blood is moving towards it, and a decrease if it is moving away.

The transducer detects an increase in frequency if the blood is moving towards it, and a decrease if it is moving away.

True or False? Doppler ultrasound imaging is an invasive technique for measuring the speed of blood flow.

False. Doppler imaging is a non-invasive technique for measuring blood flow speed in the heart or arteries.

Why does a factor of 2 appear in the Doppler ultrasound frequency shift equation?

Because the ultrasound wave travels to the moving blood cell and back again, so the Doppler shift occurs twice.

A LevelPhysicsOCRFlashcards6. Particles & Medical PhysicsUltrasound

Ultrasound (OCR A Level Physics): Flashcards

Exam code: H556

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Cards in this collection (37)

Define ultrasound.
Ultrasound is sound waves with a frequency above the human hearing range of 20 000 Hz (20 kHz).
What type of wave is ultrasound?
Ultrasound is a longitudinal wave.
Define A-scan.
An A-scan (amplitude scan) is a one-dimensional ultrasound scan used to determine the distance or depth of an internal structure.
Define B-scan.
A B-scan (brightness scan) is used to build up a two- or three-dimensional image of an internal structure, using a number of sensors or one sensor in different positions.
Why are smaller wavelengths of ultrasound used to produce clearer B-scan images?
Smaller wavelengths allow the ultrasound waves to diffract around finer points of detail on the structure, giving more detailed images.
An A-scan gives a .......... only and does not produce an image.
An A-scan gives a measurement only and does not produce an image.
True or False?
An A-scan produces a two-dimensional image of an internal structure.
False.
An A-scan gives a distance or depth measurement only; it is the B-scan that produces a two- or three-dimensional image.
Why are pulsed, rather than continuous, ultrasound waves used in B-scanning?
Pulsing allows time for the reflected wave to be received before the next pulse is transmitted, so the transmitted and reflected waves do not interfere.
Define the piezoelectric effect.
The piezoelectric effect is the ability of particular materials to generate a potential difference (p.d.) by transferring mechanical energy to electrical energy.
Define a transducer.
A transducer is a device that converts energy from one form to another.
What happens to a piezoelectric crystal when an alternating p.d. is applied across it?
It vibrates at the same frequency as the alternating p.d., generating ultrasound waves.
Why must a piezoelectric crystal be cut to a specific size?
So that it vibrates at its resonant frequency when an alternating p.d. is applied.
Why is the piezoelectric crystal in an ultrasound transducer heavily damped, for example with epoxy resin?
To stop the crystal vibrating for too long, producing short pulses and increasing the resolution of the ultrasound device.
A common piezoelectric material used in ultrasound transducers is .........., made from a lattice of silicon dioxide atoms.
A common piezoelectric material used in ultrasound transducers is quartz, made from a lattice of silicon dioxide atoms.
True or False?
A piezoelectric crystal can only transmit ultrasound, not receive it.
False.
The same crystal can act as both a receiver, converting sound waves into an alternating p.d., and a transmitter, converting an alternating p.d. into sound waves.
How does a piezoelectric crystal detect a reflected ultrasound pulse?
The returning pulse causes the crystal to vibrate, which generates an alternating p.d. across the crystal that can be processed for diagnosis.
Define acoustic impedance.
Acoustic impedance, Z, is the product of the speed of ultrasound in a medium and the density of the medium: $Z = ρ c$
State the equation for acoustic impedance, Z, and the units of each quantity.
$Z = ρ c$
where Z = acoustic impedance (kg m^-2 s^-1), ρ = density (kg m^-3), c = speed of sound in the material (m s^-1).
The greater the difference in acoustic impedance between two media, the .......... the proportion of ultrasound reflected at the boundary.
The greater the difference in acoustic impedance between two media, the greater the proportion of ultrasound reflected at the boundary.
Why does sound travel faster through solids than through gases?
Particles are closer together in solids, so vibrations can be passed between them more quickly than in a gas.
Define impedance matching.
Impedance matching is when two media have a similar acoustic impedance, resulting in little to no reflection of the ultrasound wave at their boundary.
How is the intensity reflection coefficient, α, defined?
α is the ratio of the intensity of the reflected wave to the intensity of the incident wave: $α = \frac{I_{R}}{I_{0}}$
Why is coupling gel used between an ultrasound transducer and the skin?
Gel has a similar acoustic impedance to skin, so this impedance matching minimises reflection and maximises transmission of ultrasound into the body.
True or False?
Air and soft tissue are well impedance matched, so ultrasound passes easily from air into the body.
False.
Air and soft tissue have very different acoustic impedances, so almost all the ultrasound is reflected at an air-skin boundary; coupling gel is needed to impedance match.
What property of an ultrasound wave increases the resolution of the resulting image?
A higher frequency (shorter wavelength) increases the resolution.
What two pieces of information does a reflected ultrasound pulse give about a tissue boundary?
Depth: from the time delay between transmission and receipt of the pulse.
Nature: from the amount of transmitted/reflected intensity received, which varies with tissue type.
Where in the body does ultrasound get reflected?
At boundaries between different tissues, for example between fluid and soft tissue, or tissue and bone.
Ultrasound echoes are detected by the transducer and converted into .......... signals, which are sent to the scanner.
Ultrasound echoes are detected by the transducer and converted into electrical signals, which are sent to the scanner.
What component of the transducer detects the reflected ultrasound pulse?
The piezo-electric crystal, which generates an electrical signal in response to the returning pulse.
True or False?
A lower-frequency ultrasound wave gives a higher-resolution image than a higher-frequency wave.
False.
A higher frequency (shorter wavelength) gives a higher-resolution image, allowing finer detail to be distinguished.
Define the Doppler effect.
The Doppler effect is the frequency change of a wave due to the relative motion between a source and an observer.
What happens to the observed frequency of ultrasound when the source moves towards the observer?
The frequency increases (the wavelength is shortened).
Why is Doppler ultrasound imaging effective for measuring blood flow?
Blood contains iron, which is highly reflective to ultrasound, so pulses reflected from moving blood cells can be detected.
State the equation for the frequency shift, Δf, of ultrasound reflected from a moving blood vessel, and define each symbol.
$Δ f = f_{0} - f_{r} = \frac{2 f_{0} v \cos θ}{c}$
f₀ = emitted frequency, f_r = received frequency, v = velocity of blood, θ = angle between transducer and blood vessel, c = speed of ultrasound in blood.
The transducer detects an .......... in frequency if the blood is moving towards it, and a decrease if it is moving away.
The transducer detects an increase in frequency if the blood is moving towards it, and a decrease if it is moving away.
True or False?
Doppler ultrasound imaging is an invasive technique for measuring the speed of blood flow.
False.
Doppler imaging is a non-invasive technique for measuring blood flow speed in the heart or arteries.
Why does a factor of 2 appear in the Doppler ultrasound frequency shift equation?
Because the ultrasound wave travels to the moving blood cell and back again, so the Doppler shift occurs twice.

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