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Physics 2 > C1: Transducer Construction > Flashcards

C1: Transducer Construction Flashcards

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1
Q

What is the function of a probe?

A

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

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2
Q

Example of a transducer

A 
Microphone
Oven
Ear
Light bulb
Battery
Motor
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3
Q

Parts of the probe

A 
Housing
Backing material
Crystal
Matching layer
Tuning coil
Electric shield
Insulator ring
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4
Q

3 main parts of the probe

A

Crystal
Backing material
Matching layer

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5
Q

What material is the crystal made of?

A

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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6
Q

Direct piezoelectric effect

A

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

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7
Q

Indirect piezoelectric effect

A

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

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8
Q

What are electrical dipoles?

A

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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9
Q

How are dipoles naturally arranged?

A

They are naturally random in alignment

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10
Q

How does random dipole alignment effect efficiency of the crystal?

A

It makes the crystal inefficient for vibration when current is applied

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11
Q

Why should the dipoles of a crystal be aligned?

A

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

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12
Q

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

A 
Thickness mode (good)
Radial mode
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13
Q

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

A

Thickness (good)
Length
Width

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14
Q

Why are synthetic materials used nowadays instead of crystals?

A

Synthetics produce and more pure product with less imperfections

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15
Q

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

A

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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16
Q

What is the curie temperature for PZT?

A

350 Celsius

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17
Q

What effect could reheating have on the probe?

A

It could depolarize the dipoles in the synthetic material

Probes aren’t autoclave for this reason

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18
Q

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

A

The crystal

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19
Q

What are the 4 types of frequencies?

HORD acronym

A

Resonant frequency
Driving frequency
Operating frequency
Harmonic frequency

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20
Q

Define resonant frequency, and what components effect it?

A

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

Effected by the crystal material and thickness

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21
Q

Define driving frequency and what is it effected by?

A

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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22
Q

Define Operating frequency

A

The frequency you are using to scan

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23
Q

Define 2nd harmonic frequency

A

Twice the resonant or operating frequency

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24
Q

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

A

Yes

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25
Q

What type of frequency with a thicker crystal produce?

A

Lower frequency

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26
Q

What type of frequency will a thinner crystal produce?

A

Higher frequency

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27
Q

What determines the Resonance frequency?

A

Thickness of the crystal

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28
Q

Formula that relates crystal thickness and frequency

A

Thickness = 1/2(wavelength)

29
Q

What happens to frequency when we double the crystal thickness?

A

It will half (and vice versa)

30
Q

what is Backing material?

A

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

31
Q

What’s the purpose of damping material?

A

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

32
Q

How does damping material reduce SPL?

A

By reducing the ring down

33
Q

What is the drawback of using damping material?

A

It reduces the amplitude of the wave which reduces sensitivity

34
Q

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

A

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

35
Q

What is dynamic damping?

A

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

36
Q

When is dynamic damping automatically switched on?

A

During 2D scanning

37
Q

When is dynamic damping not used and why?

A

Doppler, because you need a long spatial pulse length and high sensitivity.

Mechanical damping is used for Doppler

38
Q

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

A

Single disc mechanical probes
Annular array probes
Electronic array probes

39
Q

What are four types of mechanical probes?

A

Linear translation
Wobbler
Oscillating element
Oscillating mirror

40
Q

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

A

Electronic

41
Q

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

A

80%

42
Q

Why is a matching layer needed?

A

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

43
Q

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

A

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

44
Q

What is a potential problem with adding a matching layer?

A

Reflections can occur between the matching layer and the crystal

45
Q

How should the matching layer be cut?

A

It should be cut to 1/4 of the wavelength

46
Q

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

A

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

47
Q

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

A

Multiple layers are usually used since probes can produce multiple frequencies

48
Q

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

A

They improve the transmission and reception of wide bandwidth frequencies

49
Q

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

A

Gel

50
Q

What helps determine the driving frequency of the probe?

A

The way the probe is excited with voltage

51
Q

How were older technology probes excited with voltage?

A

Spike voltage which used a direct current

52
Q

How is the probe excited with new technology probes?

A

Burst voltage which uses alternating current

53
Q

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

A

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

54
Q

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

A

One pulse

55
Q

What is another name for DC current?

A

Saw tooth voltage

56
Q

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

A

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

57
Q

What is the appearance of a burst voltage?

A

Sine wave

58
Q

What is bandwidth?

A

The range of frequencies that are produced by a pulse

59
Q

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

A

No, there’s a very small range of frequencies that it also produces.

But the resonant frequency will have the highest amplitude.

60
Q

What effect does damping material have on the bandwidth?

A

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.

61
Q

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

A

The thickness and material of the crystal

62
Q

What’s the relationship between pulse and bandwidth?

A

Shorter the pulse, the wider the bandwidth

So Wider bandwidth means better resolution

63
Q

What’s another name for transducer?

A

Scan head

64
Q

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

A

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

65
Q

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

A

The main bang

66
Q

Q factor uses which frequency when your calculating it?

A

Center frequency or resonant frequency

67
Q

what information does Q factor give us

A

info about the efficiency of the probe

68
Q

whats an additional formula for Q factor

A

Q- Energy stored/Energy lost

69
Q

Is damping material the same and backing material?

A

Yes

Physics 2 (7 decks)

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