Define piezoelectric effect .

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

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

What happens to a piezoelectric crystal when a p.d. is applied across it, and what happens if the p.d. is reversed?

The crystal deforms ; if the p.d. is reversed, the crystal expands instead.

Why must piezoelectric crystals be cut to a specific size?

So the crystal resonates , vibrating at the same frequency as the applied alternating p.d.

One of the most common piezoelectric crystals is .......... , made from a lattice of silicon dioxide atoms.

One of the most common piezoelectric crystals is quartz , made from a lattice of silicon dioxide atoms.

True or False? A piezoelectric transducer can only transmit ultrasound waves, not receive them.

False. A piezoelectric crystal can act as both a transmitter and a receiver of ultrasound.

State two applications of the piezoelectric effect.

Microphone — detects pressure variations in sound waves and converts them into an electrical signal Ultrasound transducer — generates and detects ultrasound waves for medical imaging

Ultrasound is a high frequency sound above the range of human hearing, above 20 kHz .

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

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

What happens inside the transducer when it receives a reflected ultrasound pulse?

The piezoelectric crystal vibrates , generating an alternating p.d. (electrical signal) that is sent to the scanner.

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

Depth — the time delay between transmission and receipt of the pulse Nature — the amount of transmitted intensity received

How does the frequency of an ultrasound beam affect the scan?

The higher the frequency , the higher the resolution , so the smaller the structures that can be distinguished.

Using the speed of sound and the .......... of each echo's return, the scanner calculates the distance from the transducer to the tissue boundary.

Using the speed of sound and the time of each echo's return, the scanner calculates the distance from the transducer to the tissue boundary.

True or False? A separate detector is needed to pick up the returning ultrasound echoes.

False. The same piezoelectric crystal acts as both transmitter and receiver of ultrasound.

Define acoustic impedance .

Acoustic impedance , Z , is the product of the speed of ultrasound in a medium and the density of the medium.

In which state of matter does sound travel fastest, and why?

Sound travels fastest in solids (and slowest in gases), because the particles are closer together, allowing vibrations to move through the material faster.

What determines how much ultrasound is reflected at a boundary between two media?

The difference in acoustic impedance between the two media — the greater the difference, the greater the reflection.

Two materials with the .......... acoustic impedance would give no reflection at their boundary.

Two materials with the same acoustic impedance would give no reflection at their boundary.

True or False? Air is a suitable coupling medium between an ultrasound transducer and the skin.

False. Air has a much lower acoustic impedance than skin, so almost all the ultrasound would be reflected ; a coupling gel with a similar impedance to skin is used instead.

Define intensity reflection coefficient .

The intensity reflection coefficient , α , is the ratio of the intensity of the reflected wave relative to the incident wave.

What does a large intensity reflection coefficient at a boundary tell you about the two materials?

There is a large difference between the acoustic impedances of the two materials, so most of the ultrasound energy is reflected .

Why is a coupling gel needed between an ultrasound transducer and the body?

Air between the transducer and skin would reflect almost all the ultrasound; the gel has a similar density to skin, so little ultrasound is reflected ( impedance matching ).

The coupling gel works because it has a similar .......... to skin, meaning very little ultrasound is reflected.

The coupling gel works because it has a similar density to skin, meaning very little ultrasound is reflected.

True or False? If two materials have exactly the same acoustic impedance, half of the ultrasound intensity is reflected at their boundary.

False. If the acoustic impedances are the same, the intensity reflection coefficient is zero , so none of the ultrasound is reflected.

Define attenuation of ultrasound.

Attenuation is the reduction of energy due to the absorption of ultrasound as it travels through a material.

Generally, .......... dB cm -1 of ultrasound intensity is lost for every 1 MHz of frequency.

Generally, 0.5 dB cm -1 of ultrasound intensity is lost for every 1 MHz of frequency.

Why is attenuation not usually a major problem in ultrasound scanning?

Because the scan relies on the reflection of ultrasound at boundaries between materials, not on transmission all the way through them.

True or False? The absorption coefficient, μ , has the same value for every material.

False. The absorption coefficient varies from material to material .

A LevelPhysicsCambridge (CIE)Flashcards24. Medical PhysicsProduction & Use of Ultrasound

Production & Use of Ultrasound (Cambridge (CIE) A Level Physics): Flashcards

Exam code: 9702

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Define piezoelectric effect.

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Define piezoelectric effect.
The piezoelectric effect is the ability of particular materials to generate a potential difference by transferring mechanical energy to electrical energy.
Define transducer.
A transducer is any device that converts energy from one form to another.
What happens to a piezoelectric crystal when a p.d. is applied across it, and what happens if the p.d. is reversed?
The crystal deforms; if the p.d. is reversed, the crystal expands instead.
Why must piezoelectric crystals be cut to a specific size?
So the crystal resonates, vibrating at the same frequency as the applied alternating p.d.
One of the most common piezoelectric crystals is .........., made from a lattice of silicon dioxide atoms.
One of the most common piezoelectric crystals is quartz, made from a lattice of silicon dioxide atoms.
True or False?
A piezoelectric transducer can only transmit ultrasound waves, not receive them.
False.
A piezoelectric crystal can act as both a transmitter and a receiver of ultrasound.
State two applications of the piezoelectric effect.
- Microphone — detects pressure variations in sound waves and converts them into an electrical signal
- Ultrasound transducer — generates and detects ultrasound waves for medical imaging
Define ultrasound.
Ultrasound is a high frequency sound above the range of human hearing, above 20 kHz.
Why is the piezoelectric crystal in an ultrasound transducer heavily damped, usually with epoxy resin?
To stop the crystal vibrating for too long, producing short pulses and increasing the resolution of the device.
What happens inside the transducer when it receives a reflected ultrasound pulse?
The piezoelectric crystal vibrates, generating an alternating p.d. (electrical signal) that is sent to the scanner.
What two pieces of information does a reflected ultrasound echo give about a tissue boundary?
- Depth — the time delay between transmission and receipt of the pulse
- Nature — the amount of transmitted intensity received
How does the frequency of an ultrasound beam affect the scan?
The higher the frequency, the higher the resolution, so the smaller the structures that can be distinguished.
Using the speed of sound and the .......... of each echo's return, the scanner calculates the distance from the transducer to the tissue boundary.
Using the speed of sound and the time of each echo's return, the scanner calculates the distance from the transducer to the tissue boundary.
True or False?
A separate detector is needed to pick up the returning ultrasound echoes.
False.
The same piezoelectric crystal acts as both transmitter and receiver of ultrasound.
Define acoustic impedance.
Acoustic impedance, Z, is the product of the speed of ultrasound in a medium and the density of the medium.
State the equation for acoustic impedance.
$Z = ρ c$
where ρ is the density of the material and c is the speed of sound in the material.
In which state of matter does sound travel fastest, and why?
Sound travels fastest in solids (and slowest in gases), because the particles are closer together, allowing vibrations to move through the material faster.
What determines how much ultrasound is reflected at a boundary between two media?
The difference in acoustic impedance between the two media — the greater the difference, the greater the reflection.
Two materials with the .......... acoustic impedance would give no reflection at their boundary.
Two materials with the same acoustic impedance would give no reflection at their boundary.
True or False?
Air is a suitable coupling medium between an ultrasound transducer and the skin.
False.
Air has a much lower acoustic impedance than skin, so almost all the ultrasound would be reflected; a coupling gel with a similar impedance to skin is used instead.
Define intensity reflection coefficient.
The intensity reflection coefficient, α, is the ratio of the intensity of the reflected wave relative to the incident wave.
State the equation for the intensity reflection coefficient.
$α = \frac{I_{R}}{I_{0}} = \frac{(Z_{2} - Z_{1})^{2}}{(Z_{2} + Z_{1})^{2}}$
What does a large intensity reflection coefficient at a boundary tell you about the two materials?
There is a large difference between the acoustic impedances of the two materials, so most of the ultrasound energy is reflected.
Why is a coupling gel needed between an ultrasound transducer and the body?
Air between the transducer and skin would reflect almost all the ultrasound; the gel has a similar density to skin, so little ultrasound is reflected (impedance matching).
The coupling gel works because it has a similar .......... to skin, meaning very little ultrasound is reflected.
The coupling gel works because it has a similar density to skin, meaning very little ultrasound is reflected.
True or False?
If two materials have exactly the same acoustic impedance, half of the ultrasound intensity is reflected at their boundary.
False.
If the acoustic impedances are the same, the intensity reflection coefficient is zero, so none of the ultrasound is reflected.
Define attenuation of ultrasound.
Attenuation is the reduction of energy due to the absorption of ultrasound as it travels through a material.
State the equation for the attenuation of ultrasound intensity with distance.
$I = I_{0} e^{- μ x}$
where μ is the absorption coefficient of the material and x is the distance travelled.
Generally, .......... dB cm^-1 of ultrasound intensity is lost for every 1 MHz of frequency.
Generally, 0.5 dB cm^-1 of ultrasound intensity is lost for every 1 MHz of frequency.
Why is attenuation not usually a major problem in ultrasound scanning?
Because the scan relies on the reflection of ultrasound at boundaries between materials, not on transmission all the way through them.
On a graph of intensity in decibels against depth, how do the amplitudes of successive echoes decrease?
They decrease linearly.
True or False?
The absorption coefficient, μ, has the same value for every material.
False.
The absorption coefficient varies from material to material.

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