C1: Transducer Construction Flashcards by M B

What is the function of a probe?

It converts one form of energy to another (electric to acoustic and vice versa)

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Example of a transducer

Microphone
Oven
Ear
Light bulb
Battery
Motor

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Parts of the probe

Housing
Backing material
Crystal
Matching layer
Tuning coil
Electric shield
Insulator ring

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3 main parts of the probe

Crystal
Backing material
Matching layer

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What material is the crystal made of?

Natural materials used in early machines (quartz)
Man made materials are used today, commonly:

Lead ziconate titanate (PZT)
Polyvinylidene fluoride (PVFD)

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Direct piezoelectric effect

Mechanical pressure deforms the crystals which changes the orientation of their dipoles and produces an electrical voltage

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Indirect piezoelectric effect

An electrical voltage changes the dipoles of the crystal causing it to expand and contract

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What are electrical dipoles?

Molecules within the crystal that have a + charge at one end and a - charge at the other

They can be influenced by and electric or magnetic field

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How are dipoles naturally arranged?

They are naturally random in alignment

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How does random dipole alignment effect efficiency of the crystal?

It makes the crystal inefficient for vibration when current is applied

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Why should the dipoles of a crystal be aligned?

The vibration of the crystal will be significantly improved and the crystal will expand and contract

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What are the two modes of vibration of a crystal used in early probes?

Thickness mode (good)
Radial mode

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What 3 modes of vibration are considered for modern day probes?

Thickness (good)
Length
Width

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Why are synthetic materials used nowadays instead of crystals?

Synthetics produce and more pure product with less imperfections

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How do we make the dipoles of a substance align to enhance its piezoelectric properties?

1) heat the substance beyond its curie temperature to weaken its bonds
2) introduce an electrical field to align the dipoles
3) cool the substances and the new bonds will strengthen

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What is the curie temperature for PZT?

350 Celsius

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What effect could reheating have on the probe?

It could depolarize the dipoles in the synthetic material

Probes aren’t autoclave for this reason

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What part of the probe determines what frequencies it can emit?

The crystal

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What are the 4 types of frequencies?

HORD acronym

Resonant frequency
Driving frequency
Operating frequency
Harmonic frequency

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Define resonant frequency, and what components effect it?

The frequency the crystal likes to ring at (also called fundamental frequency)

Effected by the crystal material and thickness

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Define driving frequency and what is it effected by?

Whatever voltage is put into the crystal, or, the frequency that we can force it to ring at

Determined by the AC voltage, if changed, the crystal can be forced to ring at a different frequency than the fundamental one

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Define Operating frequency

The frequency you are using to scan

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Define 2nd harmonic frequency

Twice the resonant or operating frequency

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Is frequency of the crystal related to the propagation speed and thickness of the crystal?

Yes

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What type of frequency with a thicker crystal produce?

Lower frequency

What type of frequency will a thinner crystal produce?

Higher frequency

What determines the Resonance frequency?

Thickness of the crystal

Formula that relates crystal thickness and frequency

Thickness = 1/2(wavelength)

What happens to frequency when we double the crystal thickness?

It will half (and vice versa)

what is Backing material?

Usually and epoxy resin with metal powder that it stuck on the back of the crystal

What’s the purpose of damping material?

Reduce the spatial pulse length to improve the axial resolution AND Absorbs sound so that reflections from behind the crystal don’t occur

How does damping material reduce SPL?

By reducing the ring down

What is the drawback of using damping material?

It reduces the amplitude of the wave which reduces sensitivity

What property of the damping material allows it to stop reflections occurring from behind the crystal?

The damping material must have the same/similar Z value as the element/crystal

What is dynamic damping?

An electronic means to suppress the ringing of the crystal... one pulse is sent out and immediately after a second pulse that is slightly out of phase is sent out to stop the crystal from ringing.... this reduces the number of cycles in a pulse and increases axial resolution

When is dynamic damping automatically switched on?

During 2D scanning

When is dynamic damping not used and why?

Doppler, because you need a long spatial pulse length and high sensitivity. Mechanical damping is used for Doppler

What are the three main types of ultrasound transducers from oldest to newest?

Single disc mechanical probes Annular array probes Electronic array probes

What are four types of mechanical probes?

Linear translation Wobbler Oscillating element Oscillating mirror

If a probe is phased that means that it is in what larger category of probes?

Electronic

What percentage of sound comes back to the probe if you don’t have a matching layer?

80%

Why is a matching layer needed?

Because the impedance mismatch between the crystal and the skin is large

What is the function of the matching layer? And it is matched to what feature of the probe?

It helps sound get into the patient more efficiently, and reduces the amount of reflection.... it achieves this by having a Z value that is between that of the crystal and the skin Its matched to the frequency of the probe

What is a potential problem with adding a matching layer?

Reflections can occur between the matching layer and the crystal

How should the matching layer be cut?

It should be cut to 1/4 of the wavelength

Whats the effect of cutting the matching layer to 1/4 of the wavelength?

It helps create destructive interference of waves that reverberate between matching layers

How many matching layers are usually used? (One or multiple?)

Multiple layers are usually used since probes can produce multiple frequencies

How does having multiple matching layers effect transmission and reception of bandwidth frequencies?

They improve the transmission and reception of wide bandwidth frequencies

What’s is one example of a matching layer that is always used in ultrasound?

Gel

What helps determine the driving frequency of the probe?

The way the probe is excited with voltage

How were older technology probes excited with voltage?

Spike voltage which used a direct current

How is the probe excited with new technology probes?

Burst voltage which uses alternating current

In the spike voltage excitation method, can you change the frequency of the probe?

No... the driving/operating frequency is always equal to the resonant frequency

One voltage spike and one voltage burst is equal to.....?

One pulse

What is another name for DC current?

Saw tooth voltage

In the burst voltage mode of probe excitation, can you change the frequency?

Yes.... the driving frequency determines the frequency coming out of the probe

What is the appearance of a burst voltage?

Sine wave

What is bandwidth?

The range of frequencies that are produced by a pulse

When a crystal rings at a resonant frequency, is that the only frequency it produces?

No, there’s a very small range of frequencies that it also produces. But the resonant frequency will have the highest amplitude.

What effect does damping material have on the bandwidth?

backing material shortens the length of the pulse, and reduces amplitude of the wave so it causes a greater range of frequencies to be produced.

What determines the most efficient frequency for the crystal to ring at?

The thickness and material of the crystal

What’s the relationship between pulse and bandwidth?

Shorter the pulse, the wider the bandwidth So Wider bandwidth means better resolution

What’s another name for transducer?

Scan head

Can we use materials that don’t naturally have piezoelectric properties as a crystal element?

Yes.. we can give them piezoelectric properties by heating, applying an electric current, and then cooling them.

What is the name of the bright echo that occurs at the probe/patient interface when there is no matching layer present?

The main bang

Q factor uses which frequency when your calculating it?

Center frequency or resonant frequency

what information does Q factor give us

info about the efficiency of the probe

whats an additional formula for Q factor

Q- Energy stored/Energy lost

Is damping material the same and backing material?

Yes

C1: Transducer Construction Flashcards

(69 cards)