Define the piezoelectric effect .

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

Define a 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 when that p.d. is reversed?

The crystal deforms (compresses or stretches). If the p.d. is reversed , the crystal expands in the opposite sense.

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

So that it is cut to induce resonance , causing it to vibrate efficiently at the frequency of the applied alternating p.d.

If an alternating p.d. is applied to a piezoelectric crystal, the crystal will .......... at the same frequency as the applied voltage.

If an alternating p.d. is applied to a piezoelectric crystal, the crystal will vibrate at the same frequency as the applied voltage.

In an ultrasound piezoelectric transducer, describe the crystal's role in both generating and detecting ultrasound.

To generate ultrasound, an alternating p.d. is applied to the crystal, causing it to vibrate and produce sound waves sent into the patient's body. To detect ultrasound, returning sound waves deform the crystal, which induces a p.d. for analysis.

True or False? A piezoelectric crystal can only convert electrical energy into mechanical energy, never the reverse.

False. The piezoelectric effect works in both directions : mechanical deformation of the crystal generates a p.d., and an applied p.d. causes the crystal to deform. This is why the same crystal can both transmit and receive ultrasound.

Name two everyday applications of a piezoelectric transducer.

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

Ultrasound is a high frequency sound above the range of human hearing, generally above 20 kHz , though medical applications can use frequencies up to the MHz range.

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 , which increases the resolution of the ultrasound device.

How does increasing the frequency of an ultrasound wave affect the resulting image?

A higher frequency gives a higher resolution , allowing smaller structures to be distinguished.

The depth of a tissue boundary is found from the .......... between transmission and receipt of the ultrasound pulse.

The depth of a tissue boundary is found from the time delay between transmission and receipt of the ultrasound pulse.

What information does the intensity of a reflected ultrasound pulse provide?

The nature of the boundary, since the amount of transmitted intensity received varies depending on the type of tissue.

True or False? The piezoelectric crystal in an ultrasound transducer can only transmit ultrasound, not receive it.

False. The same crystal can act as both a transmitter and a receiver: it converts an alternating p.d. into sound waves when transmitting, and converts returning sound waves into an alternating p.d. when receiving.

Describe how an ultrasound scanner determines the distance from the transducer to a tissue boundary.

The scanner uses the speed of sound in the tissue and the time delay of the returning echo to calculate the distance travelled by the pulse.

What two properties of a material determine its acoustic impedance?

Its density and the speed of sound in the material.

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

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

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

The gel has an acoustic impedance similar to skin , so the two are impedance matched and very little ultrasound is reflected, allowing most of it to be transmitted into the body.

Define impedance matching .

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

True or False? If two materials have the same acoustic impedance, none of the ultrasound will be transmitted through the boundary.

False. If the acoustic impedances are equal, the intensity reflection coefficient is zero , so there is no reflection and the ultrasound is fully transmitted.

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.

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

A B-scan is an ultrasound scan 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.

Give one clinical use of an A-scan.

Determining the length of the eye , for example when planning surgery or assessing for abnormalities such as tumours.

How is a B-scan image built up from a series of measurements?

The transducer is moved over the patient's skin , or several transducers are used, to produce a series of A-scans in different positions, which are combined to form an image.

Ultrasound cannot penetrate .......... or gas, as these reflect or dampen the sound waves.

Ultrasound cannot penetrate bone or gas, as these reflect or dampen the sound waves.

Give two advantages of ultrasound compared with MR or CT scanning.

Cheaper, faster and more portable Involves no ionising radiation Non-invasive Produces real-time images Patients do not need to remain completely still

True or False? Ultrasound can distinguish between benign and malignant tumours.

False. Ultrasound cannot distinguish between benign and malignant tumours, as both are solids with different acoustic impedances that reflect ultrasound similarly. Unlike PET scans, ultrasound cannot make this distinction.

Why are smaller wavelengths used in ultrasound imaging to give more detailed images?

Smaller wavelengths allow the sound waves to diffract around finer points of detail on the internal structure being studied, revealing more detail.

Define a coherent bundle of optical fibres.

A coherent bundle is a bundle of optical fibres with fixed positions relative to each other at each end and along its length; it is used to transmit an image to the viewer.

Define an incoherent bundle of optical fibres.

An incoherent bundle is a bundle of optical fibres grouped together in a random arrangement; it is used to transmit light from a source to illuminate the target tissue.

Why does the cladding of an optical fibre have a slightly lower refractive index than the core?

So that light travelling in the core meets the boundary at an angle greater than the critical angle , allowing it to undergo total internal reflection .

Why are fibres in a coherent bundle not made narrower than about 10 μm in diameter?

Below this diameter, diffraction begins to affect the image quality.

An incoherent bundle is used to .......... the target tissue, rather than to transmit an image.

An incoherent bundle is used to illuminate the target tissue, rather than to transmit an image.

Name the components contained within the flexible shaft of an endoscope.

An aperture for operating medical instruments A channel for air or water An incoherent bundle for illumination A coherent bundle for transmitting an image

True or False? Endoscopy is always preferable to surgery for examining internal body systems.

False. Endoscopy is limited to small procedures and certain systems; sometimes surgery, magnetic resonance scans or other forms of imaging are more appropriate.

Give three benefits of endoscopy compared with surgery.

Less painful than surgery Lower infection risk than surgery Faster recovery time than surgery

Define nuclear magnetic resonance (NMR).

Nuclear magnetic resonance occurs when a proton absorbs a photon of exactly the energy required to flip its spin from a lower energy state to a higher energy state.

Why are hydrogen nuclei used in MRI scanning?

The body contains more hydrogen nuclei (protons) than any other element, so aligning their magnetic fields allows nuclear magnetic resonance to be observed throughout the body's tissues.

What happens to the spin states of protons when a magnetic field is applied?

A difference in energy arises between the two spin states: the parallel (spin up) state is lower energy, and the antiparallel (spin down) state is higher energy. Most protons occupy the lower energy state, producing a net magnetic moment.

When a magnetic field is applied, spinning protons .......... about the direction of the field, like a spinning top.

When a magnetic field is applied, spinning protons precess about the direction of the field, like a spinning top.

What is the purpose of the gradient field in an MRI scanner?

It varies the magnetic flux density with position, so hydrogen nuclei at different positions emit RF photons of different frequencies , allowing their position to be determined.

Give two advantages of MRI scanning.

Non-ionising and non-invasive Produces extremely high-resolution images Can diagnose very small differences between cells, e.g. cancerous cells

True or False? An MRI scan shows how much an organ is functioning.

False. MRI scanning shows structures in the body, such as organs and masses like tumours, but not how much they are functioning.

Give two disadvantages of MRI scanning.

Time-consuming and can be uncomfortable for patients Very expensive

A LevelPhysicsAQAFlashcardsMedical PhysicsNon-Ionising Imaging

Non-Ionising Imaging (AQA A Level Physics): Flashcards

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FrontThe Piezoelectric Transducer
Define the piezoelectric effect.

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Cards in this collection (47)

Define the piezoelectric effect.
The piezoelectric effect is the ability of particular materials to generate a potential difference by transferring mechanical energy to electrical energy.
Define a 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 when that p.d. is reversed?
The crystal deforms (compresses or stretches). If the p.d. is reversed, the crystal expands in the opposite sense.
Why must a piezoelectric crystal be cut to a specific size?
So that it is cut to induce resonance, causing it to vibrate efficiently at the frequency of the applied alternating p.d.
If an alternating p.d. is applied to a piezoelectric crystal, the crystal will .......... at the same frequency as the applied voltage.
If an alternating p.d. is applied to a piezoelectric crystal, the crystal will vibrate at the same frequency as the applied voltage.
In an ultrasound piezoelectric transducer, describe the crystal's role in both generating and detecting ultrasound.
To generate ultrasound, an alternating p.d. is applied to the crystal, causing it to vibrate and produce sound waves sent into the patient's body. To detect ultrasound, returning sound waves deform the crystal, which induces a p.d. for analysis.
True or False?
A piezoelectric crystal can only convert electrical energy into mechanical energy, never the reverse.
False.
The piezoelectric effect works in both directions: mechanical deformation of the crystal generates a p.d., and an applied p.d. causes the crystal to deform. This is why the same crystal can both transmit and receive ultrasound.
Name two everyday applications of a piezoelectric transducer.
- Microphone — detects pressure variations in sound waves and converts them to an electrical signal
- Ultrasound — generates and detects ultrasound waves for medical imaging
Define ultrasound.
Ultrasound is a high frequency sound above the range of human hearing, generally above 20 kHz, though medical applications can use frequencies up to the MHz range.
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, which increases the resolution of the ultrasound device.
How does increasing the frequency of an ultrasound wave affect the resulting image?
A higher frequency gives a higher resolution, allowing smaller structures to be distinguished.
The depth of a tissue boundary is found from the .......... between transmission and receipt of the ultrasound pulse.
The depth of a tissue boundary is found from the time delay between transmission and receipt of the ultrasound pulse.
What information does the intensity of a reflected ultrasound pulse provide?
The nature of the boundary, since the amount of transmitted intensity received varies depending on the type of tissue.
True or False?
The piezoelectric crystal in an ultrasound transducer can only transmit ultrasound, not receive it.
False.
The same crystal can act as both a transmitter and a receiver: it converts an alternating p.d. into sound waves when transmitting, and converts returning sound waves into an alternating p.d. when receiving.
Describe how an ultrasound scanner determines the distance from the transducer to a tissue boundary.
The scanner uses the speed of sound in the tissue and the time delay of the returning echo to calculate the distance travelled by the pulse.
Define acoustic impedance.
Acoustic impedance, Z, is the product of the speed of ultrasound in a medium and the density of that medium: $Z = ρ c$
What two properties of a material determine its acoustic impedance?
Its density and the speed of sound in the material.
The greater the .......... in acoustic impedance between two media, the greater the proportion of ultrasound reflected at their boundary.
The greater the difference in acoustic impedance between two media, the greater the proportion of ultrasound reflected at their boundary.
Why is a coupling gel used between an ultrasound transducer and the skin?
The gel has an acoustic impedance similar to skin, so the two are impedance matched and very little ultrasound is reflected, allowing most of it to be transmitted into the body.
Define impedance matching.
Impedance matching occurs when two media have a similar acoustic impedance, resulting in little to no reflection of the ultrasound wave at their boundary.
State the equation for the intensity reflection coefficient, α, in terms of the acoustic impedances Z₁ and Z₂.
$α = \frac{I_{r}}{I_{0}} = \frac{(Z_{2} - Z_{1})^{2}}{(Z_{2} + Z_{1})^{2}}$
True or False?
If two materials have the same acoustic impedance, none of the ultrasound will be transmitted through the boundary.
False.
If the acoustic impedances are equal, the intensity reflection coefficient is zero, so there is no reflection and the ultrasound is fully transmitted.
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.
Define an A-scan.
An A-scan is a one-dimensional ultrasound scan used to determine the distance or depth of an internal structure.
Define a B-scan.
A B-scan is an ultrasound scan 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.
Give one clinical use of an A-scan.
Determining the length of the eye, for example when planning surgery or assessing for abnormalities such as tumours.
How is a B-scan image built up from a series of measurements?
The transducer is moved over the patient's skin, or several transducers are used, to produce a series of A-scans in different positions, which are combined to form an image.
Ultrasound cannot penetrate .......... or gas, as these reflect or dampen the sound waves.
Ultrasound cannot penetrate bone or gas, as these reflect or dampen the sound waves.
Give two advantages of ultrasound compared with MR or CT scanning.
- Cheaper, faster and more portable
- Involves no ionising radiation
- Non-invasive
- Produces real-time images
- Patients do not need to remain completely still
True or False?
Ultrasound can distinguish between benign and malignant tumours.
False.
Ultrasound cannot distinguish between benign and malignant tumours, as both are solids with different acoustic impedances that reflect ultrasound similarly. Unlike PET scans, ultrasound cannot make this distinction.
Why are smaller wavelengths used in ultrasound imaging to give more detailed images?
Smaller wavelengths allow the sound waves to diffract around finer points of detail on the internal structure being studied, revealing more detail.
Define a coherent bundle of optical fibres.
A coherent bundle is a bundle of optical fibres with fixed positions relative to each other at each end and along its length; it is used to transmit an image to the viewer.
Define an incoherent bundle of optical fibres.
An incoherent bundle is a bundle of optical fibres grouped together in a random arrangement; it is used to transmit light from a source to illuminate the target tissue.
Why does the cladding of an optical fibre have a slightly lower refractive index than the core?
So that light travelling in the core meets the boundary at an angle greater than the critical angle, allowing it to undergo total internal reflection.
Why are fibres in a coherent bundle not made narrower than about 10 μm in diameter?
Below this diameter, diffraction begins to affect the image quality.
An incoherent bundle is used to .......... the target tissue, rather than to transmit an image.
An incoherent bundle is used to illuminate the target tissue, rather than to transmit an image.
Name the components contained within the flexible shaft of an endoscope.
- An aperture for operating medical instruments
- A channel for air or water
- An incoherent bundle for illumination
- A coherent bundle for transmitting an image
True or False?
Endoscopy is always preferable to surgery for examining internal body systems.
False.
Endoscopy is limited to small procedures and certain systems; sometimes surgery, magnetic resonance scans or other forms of imaging are more appropriate.
Give three benefits of endoscopy compared with surgery.
- Less painful than surgery
- Lower infection risk than surgery
- Faster recovery time than surgery
Define nuclear magnetic resonance (NMR).
Nuclear magnetic resonance occurs when a proton absorbs a photon of exactly the energy required to flip its spin from a lower energy state to a higher energy state.
Why are hydrogen nuclei used in MRI scanning?
The body contains more hydrogen nuclei (protons) than any other element, so aligning their magnetic fields allows nuclear magnetic resonance to be observed throughout the body's tissues.
What happens to the spin states of protons when a magnetic field is applied?
A difference in energy arises between the two spin states: the parallel (spin up) state is lower energy, and the antiparallel (spin down) state is higher energy. Most protons occupy the lower energy state, producing a net magnetic moment.
When a magnetic field is applied, spinning protons .......... about the direction of the field, like a spinning top.
When a magnetic field is applied, spinning protons precess about the direction of the field, like a spinning top.
What is the purpose of the gradient field in an MRI scanner?
It varies the magnetic flux density with position, so hydrogen nuclei at different positions emit RF photons of different frequencies, allowing their position to be determined.
Give two advantages of MRI scanning.
- Non-ionising and non-invasive
- Produces extremely high-resolution images
- Can diagnose very small differences between cells, e.g. cancerous cells
True or False?
An MRI scan shows how much an organ is functioning.
False.
MRI scanning shows structures in the body, such as organs and masses like tumours, but not how much they are functioning.
Give two disadvantages of MRI scanning.
- Time-consuming and can be uncomfortable for patients
- Very expensive

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